First version of Calypso Stats

The UI still need to be refined, but major features are all there. 
We want to start testing new Stats for real asap, and meanwhile refine stats UI with smaller PRs.
There are 11 checks for `if (mDatamodels == null || mDatamodels[0] == null)` in the various stats fragments. How about a single method (perhaps `isDataEmpty()`) in StatsAbstractListFragment that can be called to check for empty data instead?