make embargo template theme-able

Use "Stanford-style" theming to enable you to put your own embargo
notice up.
missing themed embargo page fails safely to unthemed
Switch from Rake to Paver
Change `if not args.skip_collect` to `if args.collect`

Simpler and clearer
from __future__ import print_function

and print errors and warnings to sys.stderr
write_stderr() function is unnecessary
Pull Paver requirements out into a separate file
Insert *right* at the front
Documentation for static page changes

STUD-1347
Merge pull request #3005 from singingwolfboy/db/paver

Paver v3
Merge pull request #2928 from edx/markhoeber/documentation/stud-1347

Documentation for changes to static pages
Help paver find the theme directory
Merge pull request #3075 from edx/db/paver-find-theme-dir

Help paver find the theme directory
fix paver theming on devstack (SERVICE_VARIANT support)

When running in devstack, you need to set the SERVICE_VARIANT so that
assets.py knows what environment file to use.  Recall that in devstack
we have all our settings in ../lms.env.json, not just ../env.json.  So
you need to run like this:

    SERVICE_VARIANT=lms paver lms --settings=devstack

Previously you'd get this error:

    Warning: could not find environment JSON file at 'lms.env.json'

Note there is no path on the file, that's the bug.  By making
self.REPO_ROOT absolute, we can walk up the tree and correctly find
the file, which is /edx/app/edxapp/lms.env.json fwiw.
Merge pull request #3081 from edx/sef/paver-fix-devstack-theme

fix paver theming on devstack (SERVICE_VARIANT support)
Merge branch 'sef/theme-embargo-master' of github.com:edx/edx-platform