implemented correct identities for PUT

Assigned profile_name to be the identifier for EncodedVideos.
Tests for PUT have been added to check for expected behavior.
Attempting to PUT multiple EncodedVideos with the same profile
will return an error.

Other notes:
-url now accepts dashes
-reduced number of unique constants
-updated api.py tests with HTTP requests
-removed unused repr
Merge pull request #9 from edx/clee/put

tests for PUT functionality