Revert exponents-radicals to non-empty file.

Got deleted due to no longer existing on KA site.

Need their API to match to their live data.
Tweak to wording on facility edit page (thanks Rui!).
Video thumbnails now display for related videos on exercises.
Update to .gitignore and instruction to put independently downloaded
content in content folder.
Add Mac OS X installation instructions.
Merge upstream
Merge pull request #17 from juliaogris/develop

OSX installation instructions
Fixed universal subtitle API download URL.
Added helper function for listing downloaded video youtube ids.
Updated the topic tree. Now in line with the first torrent file's videos.
Merge branch 'develop' of github.com:jamalex/ka-lite into develop
Use .png image for all thumbnail contexts; don't need the .jpg anymore.
Merge branch 'develop' of github.com:jamalex/ka-lite into develop