updating to build 81
fixes for lesson downloading problems
now support Android DownloadManager
fixes for rendering and downloading
rendering now works on external storage again (preference)
small fixes to UI navigation issues (home/back)
other small UI bug fixes
makes sure the camera is available`
handles bad settings input
disabled automated check for update (so we can release in google play)
updated version
updating to build 81
Merge branch 'bugs-downloading-rendering'

Conflicts:
	app/AndroidManifest.xml