Feature/mixpanel. Adds stats tracking for mixpanel for posting related activites. 

This is a first pass at adding stats tracking via mixpanel to the android app and its ready for a code review.
The implementation is patterned after @sendhil's approach on the iOS app.  

A few caveats :
- The mixpanel token should to be added to your copy of local.properties when building. 
- Only posting actions are tracked. 
- Since this is a first pass at the implementation, not everything that could be tracked is. We'll improve the actions being tracked post code review. 

Sendhil mentioned there are some improvements recommended by the mixpanel folks to the way we're tracking various things so I anticipate we'll need to adjust the way some properties and events being tracked.

Refs #288