Added Today statistics, removed achartengine dependence from smal due chart

I removed dependence of achartengine of the small due chart, some dependence remains I look after it next. But more than that I bundled all chart engine creation now in the new ChartBuilder class. Before it was split at two places, which I found hard to find. So even when the chartengine is changed again/back that should work better I think.

Today stats are now added. The fragment containing it looks a bit empty though.
I thought about making a Statistics Overview Fragment out of it where I display first the Today statistics and then a list (with maybe icons of the charts but without charts graphs itself) of all charts with their subtitle and extra informations that are not on the charts themselves (every element then links to the corresponding tab).
Those informations are below the charts on Anki-Desktop but I don't know if that makes sense to recreate on Android because of smaller devices. But in the WebView it would be comfortable and scrollable also I could recreate the same look like on desktop because there html is used for everything in statistics. 

So I could really need some feedback/input for that right now.

Oh yes also it would be nice to know how or where to integrate the "collection, deck" and "1 month, 1 year, deck life" radio buttons. 
I could create a dialog created from an item in the options menu or I could put those options on the bottom of the proposed Overview Fragment to look a bit like AnkiDesktop but that probably is not Android style. AnkiWeb has them on top if I have seen correctly but that also is not Android style.
Do you mind changing to switch/case syntax? It's much cleaner, and that's how we implement  `onOptionsItemSelected` in all the other activities...