`rake install_prereqs` requires a minimal level of ruby and rake already
installed. If it doesn't exist, then print out a helpful message
indicating next steps.

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

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.

Move Mysql and scipi to requirements/post.txt, because they require the contents of requirements.txt to be completely installed

soon anyway).  Removed @skip-phantom and @skip-firefox tags

Fixed sign up test to work in Ubuntu

Added comments

moved syncdb/migration steps into the rake test_acceptance commands
and updated the documentation.

Because removing a step from our install process is a Good Thing

Updated testing docs to reflect changes to the rake file

Added option to pass arguments (including --pdb) to rake test_acceptance
Updated readme

all acceptance tests.

Add a private-requirements.txt for personal changes to how things are installed.  Good for installing libs as working dirs.