Add support for MapBox tiles instead of Google Maps

Added support for MapBox images.

To use, include MAPBOX_API_KEY in Config.java.

The Themes.xml now has mapSource which can be set to 'google' or 'mapbox' - the existing 2 themes have been updated and set to google. A new Mapbox theme has been added.

MapTheme now reads the source property (and defaults to Google for compat if none is found), and then reads styles for Google Maps, or reads the mapId for MapBox.

The source is set on the theme and MapImage returns URLs as appropriate.

![screenshot_2014-12-12-00-38-24](https://cloud.githubusercontent.com/assets/1222909/5407602/ef0ed762-817e-11e4-85fb-1ad61fa0462b.png)
![screenshot_2014-12-12-00-38-02](https://cloud.githubusercontent.com/assets/1222909/5407601/ef0ea8fa-817e-11e4-8067-20abc5a18613.png)