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.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into develop
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/static/js/videoplayer.js
	kalite/templates/video.html
updating based on code reviews, more fully merging with existing changes
fixing merge bug, moving code from template to static file
Merge branch 'release-0.10' into mplayer_launcher

Conflicts:
	kalite/main/api_views.py
	kalite/main/views.py
	kalite/settings.py
	kalite/static/js/videoplayer.js
	kalite/templates/video.html
	kalite/utils/internet.py
Merge branch 'mplayer_launcher' of https://github.com/bcipolli/ka-lite into mplayer_launcher

Conflicts:
	kalite/main/api_views.py
	kalite/main/views.py
	kalite/static/js/videoplayer.js
	kalite/templates/video.html
Fixed up background saving of points for mplayer launcher.
Merge remote-tracking branch 'upstream/release-0.10' into mplayer_launcher
Merge branch 'release-0.10' into mplayer_launcher
Only allow USE_MPLAYER to be True when CACHE_TIME == 0, for now.
Merge branch 'release-0.10' of github.com:learningequality/ka-lite into mplayer_launcher

Conflicts:
	kalite/main/api_views.py
Add headers in the Nginx config to pass the remote IP address to Django.
Make the is_loopback_connection more constrained and robust.