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.
Normally the files in libanki folder are expected to follow the corresponding Python file in Anki Desktop quite closely --  e.g. [stats.py](https://github.com/dae/anki/blob/master/anki/stats.py)
@hssm what do you think?