Paver (Part 1)

This is a rebased and updated version of #2146.  It contains the first set of Paver commands to replace Rake commands for:

* Installing and updating Python/Ruby/Node prereqs
* Compiling and collecting assets
* Running local dev servers (including devstack and celery workers)
* Building documentation

This PR does not contain the Paver commands for tests, quality, or i18n, as these are currently being actively changed.  When those commands stabilize, a future PR will update and merge the Paver commands for these tasks from #2146 .

In consultation with folks from the LMS, Studio, and Doc teams, I have been more aggressive about deprecating unused commands.  In particular, this PR goes further than #2146 by deprecating:

* Asset watchers (there are plans to add a new system soon to implement this functionality)
* Showing documentation in a web browser (the docs team doesn't use it)
* Running Celery workers with Studio settings (Studio doesn't currently have Celery tasks)
* Course management commands (setting staff bits, importing courses, etc.)  Studio team uses manage.py directly for this.

I have also taken the opportunity to replace "cms" in the Paver commands with "studio", on the assumption that this will be easier for open-source contributors.  I have not changed the manage.py commands, so this shouldn't have an operational impact.  In particular, the `gather_assets` rake task (used by deploy scripts) still accepts "cms" and works correctly.

I have verified that this branch successfully deploys to a sandbox both with and without Stanford-style theming, without requiring changes to the configuration scripts.

Huge thanks to @dglance for taking on this enormous task!  I hope to have the remaining parts of the original PR updated and merged soon as well.

Reviewers:
@jzoldak 
@dianakhuang 
@jbau