added POST/GET for video models

Initial part of the upload portion of the API. Video Models can
be uploaded via POST as a list of dictionaries. Matching entries
will be updated and new entries will be created. Errors are
returned in a list of tuples with the message and input. Single
entries can be returned by edx_video_id via GET.

Other notes:
-Django Rest Framework upgraded for APIView 2.3.5->2.3.14
-Field "client_title" -> "client_video_id" in the Video Model
POST EncodedVideo set

EncodedVideo part of the upload. Complete EncodedVideo dicts may
now be uploaded. Rejects all EncodedVideos of a Video when there is
an invalid EncodedVideo. Does not reject other Video when a differet
Video is invalid. Currently, EncodedVideo cannot be updated, they can
only be created.

Other notes:
-Added django-debug-toolbar==1.2.1
-constants.py has been reorganized