Added option to launch mplayer in the background to play videos.

The Raspberry Pi browsers cannot yet play video. But mplayer works on the
Pi, so we can trigger it to be launched when the user wants to view a
video. Point tracking is done in the backend Python thread that launched
mplayer, and the browser refreshes the point total when mplayer is closed.

Add `USE_MPLAYER = True` to local_settings.py to enabled this. Note that
it will only work when the browser is on the same computer as the server.
Merge branch 'develop' of github.com:learningequality/ka-lite into mplayer_launcher

Conflicts:
	kalite/main/api_views.py
	kalite/main/views.py
	kalite/settings.py
	kalite/templates/video.html
	kalite/utils/internet.py
Tweaks and commenting to address mplayer code review.
Use getattr syntax for USE_MPLAYER setting, to allow overriding.
Add name to launch_mplayer api url, so it can be reversed.
Fixes to mplayer thread launching.
To prevent multiple mplayer instances, throttle the launch clicks.
move as much installation logic to python as possible.
migrate from print/sys.exit to stdout/CommandError. Get database path programmatically. Configure permissions for apache in apache config script.
migrate from print/sys.exit to stdout/CommandError. Get database path programmatically. Configure permissions for apache in apache config script. Make noinput / non-interactive option.
Merge branch 'install_via_python' of github.com:bcipolli/ka-lite into install_via_python
fixing misspelling.
fixing bug in setting the admin password.
small changes for self code-review
fixing daemon bug, small bugs from testing
Merge branch 'develop' of github.com:learningequality/ka-lite into install_via_python

Conflicts:
	kalite/utils/general.py
remove infinite loops from management commands. remove cruft from api_views, and tweak video_download to handle the more robust videodownload logic
updating comments, debug-based Exception forcing, to complete.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into avoid_command_infinite_loops

Conflicts:
	kalite/templates/video_download.html
Fix the error with exercise description and added a test class for all urls data structure.