URL change: /video -> /videos, added /admin

Changed the resource URLs to be plural for consistency with other
services in edx-platform. Added /admin for standalone running with
"manage.py runserver". Instead of using edxval.urls directly,
runserver now uses a top level urls.py that includes edxval.urls
as well as the debug toolbar and admin.
Merge pull request #22 from edx/ormsbee/fix_urls

URL change: /video -> /videos, added /admin