Adds a feature to the edX platform which allows instructors to set
individual due dates for students on particular coursework. This code is
meant primarily for on-campus use--it is not intended that this feature
would be used for MOOCs. It adds a new tab, "Extensions", to the beta
instructor dashboard which allows changing due dates per student. This
feature is enabled by setting FEATURES['INDIVIDUAL_DUE_DATES'] = True.

Cf README, this adds support to install and setup a dev instance through
Vagrant. The Vagrantfile at the root handles the vagrant commands like
`vagrant up`.

Provisioning is done by the shell script `vagrant-provisionning.sh`,
which is a light wrapper around the `create-dev-setup.sh` which does
most of the job using the standard process, but limiting possible
side-effect by ensuring the environment on which the install is
performed is clean.

Based on an Ubuntu precise 12.04 LTS.

Developers can have private settings files by creating
lms/envs/private.py or cms/envs/private.py.  They are imported
at the end of dev.py.  Note that they won't be imported if you
are using one of the other dev*.py variants.

Just as is done with the main LMS application.scss file, rewrite the
course.scss file with Mako to conditionally import a theme's
variables overrides. Add the course.scss file to the list of ignored
Git files so that it doesn't keep getting committed over and over
again.

This also requires us to add a hardcoded line in the assets Rakefile
for the moment, so that the course.scss.mako file gets properly
preprocessed. Once the preprocessing is done by a Django management
command, we won't have to do this anymore.

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.

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

Tag tests not working under PhantomJS to skip for now.

This reverts commit 793354a0.