Bump minSdk to 15

Supports roughly 85% of Android devices as of 2014/08/05, which should
be more than adequate, plus it grants us access to more API features.
Add the values xml resources
Add MozHelper
Add the model classes
Add RankAdapter for leaderboard's data
Add the drawable resources
Add StatsAdapter for leaderboard's data
Add TabBarFragment
Add Mapbox SDK
Add MapFragment
Add LeaderboardFragment
Add SettingsFragment
Add MozStumblerActivity
Use MozStumblerActivity as main activity
Update app theme styling for the new UI
Hardware acceleration is required

While enabling will use more memory, this is a map-based game, so
performance is a fairly high priority.
Refactor MozStumblerActivity into MainActivity

- Remove developer overlay
- Add StumblingDataFragment
- Add power button
- Add data button
- Add zoom to self button
Persist data in StumblingDataFragment