Add features to API

Changes:
- Update API docs for features to drop extended feature relations
- Add /api/v1/features endpoint
- Add caching, models, and fields needed to support endpoint
- Add basic browse page for feature data
- Improve `tools/load_webcompat_data.py` to load subset of feature data (default) or all feature data
- Update Django to 1.6.7
- Update pylibmc to Python 3-friendly commits

Some additional work has been deferred until all the resource endpoints are added:
- https://bugzilla.mozilla.org/show_bug.cgi?id=1078682 - Query param include=[relation] includes linked data
- https://bugzilla.mozilla.org/show_bug.cgi?id=1078699 - Resources available by slug