Initial migration for timestamp fields.
Merge branch 'develop' into feature/2767-add-timestamp-fields-to-contentLog-model

* develop:
  update youtube_dl to 2014.12.10.3 release
  save 5
  save 4
  save 3
  delete khan-exercises directory and changes the path of the 'khan-exercises' to 'perseus/ke' static path.
  save 2
  save 1
Initial fix for the timestamp field.
- Add start-timestamp handlebar file for the time start on watching the video.
Added ContentLog filter fields in django admin.
Modify ContentLog to save progress_timestamp in main/models.py.
Modify list_filter in main/admin.py
Modify save method in ContentLog.
Revert main/api_views.py, content/models.js, views.js, and video player.js.
Revert content/models.js, views.js and video player.js.
Remove start-timestamp.handlebars.
Initial implementation of test in progress_timestamp and start_timestamp.
Modify the progress_timestamp_test.py, making sure that the start_timestamp will not change, and progress_timestamp will change when we update contentlog
Change assertNotEqual to assertTrue in progress_timestamp_test.py
Merge pull request #2790 from mrpau/feature/2767-add-timestamp-fields-to-contentLog-model

Feature/2767 add timestamp fields to content log model