WIP: mattdrayer/api-position-tree: Current state (debugging)

@chrisndodge -- here's current state of the position tree implementation -- here's the CLI command to get it started:

rake fasttest_lms[lms/djangoapps/api_manager/users/tests.py:UsersApiTests.test_user_courses_detail_post_position_course_as_descriptor]

I've also confirmed that the DRF web UI does not throw errors and results in the same behavior, which is strangely comforting...