Django allows forms and widgets to define the CSS or JavaScript files
needed on the page, automatically including those files in the
administration UI. This works fine for standalone media files, but
weren't an option for Pipeline packages.

This change introduces a branch new `PipelineFormMedia` class that a
form or widget's Media class can inherit from. The Media class can then
define `css_packages` or `js_packages` attributes that work just like
the standard `css`/`js` attributes but reference package names instead
of individual files.

Upon accessing the `css`  or `js` attributes, the files defined in the
packages will be collected/processed just like with the template tags
and the resulting filenames returned for use in the page.

Using this, any page compatible with Django's form media support will
work automatically with any form or widget that wants to use Pipeline
packages, without any modifications to templates or any other
workarounds that were needed before.

This implements issue #154.