Documentation

Ref https://github.com/mapbox/www.mapbox.com/issues/4126

This commit add's a `docs` directory to the main repo and creates online developer documentation for Mapbox Android SDK. To generate the API docs I've added instructions here: https://github.com/mapbox/mapbox-android-sdk/tree/styled-docs/docs

There's a couple pieces left to make this real 

- Move the master default branch to `mb-pages` 
- Set up publisher to update changes.
- Add `mapbox-android-sdk` to the sync-templates list.

Ready for review @bleege @tmcw