This resolves an error that would sometimes occur on Mac OS X:
"Error: EMFILE, too many open files"

Previously, multiple copies of the watchers started from the different
shells would run simultaneously, which left the possiblity of zombie
watchers, increased resource consumption, and incorrect results. This
fixes that problem by only starting a watcher if that same command isn't
already in the process list.

Fixes LMS-499

When running under watchmedo, coffee doesn't display any useful
information when it recompiles a changed file, so we make watchmedo echo
that information instead.

Using `ulimit -n` to set the limit much higher than the default of 256
in Darwin seems to avoid the `EMFILE` error that was plaguing our Mac
developers.

The preprocess task requires system and env arguments in order to
correctly load up the django environment to preprocess sass files to
inject themes.

In order for that task to recieve the arguments, all tasks that depend
on it also have to accept that same set of arguments.

This will all go away once the next evolution of themes arrives, which
will remove the preprocessing needed to inject theme names.

Rake by default imports all .rake files in the rakelib dir, so we can
use that rather than doing our own import loop.

This prevents inapplicable tests from running when switching branches

If no default is provided, sass falls back to the system
default. This fails when the system default is not unicode because
various sass files have unicode characters in them.

Use coffee watch command to watch whole directories, so that it will pick up new coffee files as they are created

Using `task()` reopens the definition of the task, and all we really
need to do is get a reference to the task itself to invoke it.

Rather than directly invoke command-line Python (and Mako) from the
assets Rakefile, or call an external Python script, use a Django
management command to preprocess all asset template files.

An "asset template file" is defined as a static asset file with a
file extension indicating that it needs to be run through a template
engine prior to Sass/CoffeeScript compilation or packaging with other
assets. The preprocess_assets management command will look through
all of the files listed in the `STATICFILES_DIRS`, preprocessing each
as needed. Preprocessing strips off the special template file
extension, creating a new file in the process.

Currently, the only variable accessible in an asset template file is
the `THEME_NAME`, defined in the settings.

Capture the exit code of the Mako template engine invocation on
asset preprocessing and abort from the Rake task on failure. This
will prevent the LMS from continuing its attempt to start up,
preventing further configuration errors.

Per feedback from @cpennington. This way we'll only preprocess once,
even if the Sass compilation is invoked multiple times.

This commit adds the requisite settings and startup features to
enable integration of themes into the edX platform. It does not
yet provide hooks in any of the templates, but it does cause the
main `lms/static/sass/application.scss` file to `@import` a theme's
base Sass. Template hooks will come down the road.

CHANGELOG
---------
Define a new `MITX_FEATURE`, `USE_CUSTOM_THEME`, that when enabled,
can be used in templates to determine whether or not custom theme
templates should be used instead of the defaults.

Also define a new setting, `THEME_NAME`, which will be used to
locate theme-specific files. Establish the convention that themes
will be stored outside of the `REPO_ROOT`, inside the `ENV_ROOT`,
in a directory named `themes/`. `themes/<THEME_NAME>` will store
the files for a particular theme.

Provide a function, `enable_theme`, that modifies the template and
static asset load paths appropriately to include the theme's files.

Move the main LMS Sass file to a Mako template that conditionally
`@import`s the theme's base Sass file when a theme is enabled.

Add logic to the assets Rakefile to properly preprocess any Sass/
Mako templates before compiling them.

This is to work around an OSx issue that causes EMFILE errors in the
default configuration.
Ref: https://github.com/joyent/node/issues/2479

Make the externally facing asset watch commands wait for user intervention, and add internal tasks that only run as long as the rake process