Added rake commands for devstack

An attempt to simplify running LMS/Studio in the devstack vagrant image.

    # Start LMS/Studio (including asset collection and installing prereqs)
    rake devstack[lms]
    rake devstack[studio]

    # Start LMS/Studio skipping asset collection/prereqs
    rake devstack:start[lms]
    rake devstack:start[studio]

Some of the controversial decisions:

* Replacing "cms" with "studio", for consistency with the rest of the devstack documentation.
* Not using Sass watchers, on the assumption that the design team prefers custom tools like CodeKit

Currently, prereq installation is disabled in devstack by an environment variable.  I've put in a PR to change this:
https://github.com/edx/configuration/pull/548  Until then, you'll need to `unset NO_PREREQ_INSTALL` so that prereqs get installed.

Suggested reviewers:

@cpennington for rake file sanity checks
@flowerhack since you asked for simpler devstack commands :)