New statistics to include all graphs from Anki-Desktop

I implemented all graphs from Anki-Desktop statistics plus one extra (Weekly Breakdown).
I used a chart engine that I made myself. When I discover a way to use achartengine for the cumulative graphs I could switch back if wanted. There a some cosmetic issues with disappearing axis-labels when numbers are getting too big. Also some pixel adjustments for the legend and ticks are necessary.
Since I wanted to implement the navigation drawer for now and so use the newest NavigationDrawerActivity I thought it best to send a pull request.
Does this stuff in DeckTask actually do anything? Since you are implementing your own `AsyncTask` I think DeckTask should basically not include anything to do with statistics...?