Mbtiles

Actually it's a lot more than just MBTiles support.

It's a refactoring of ITileLayer to support all kind of sources.

As a result that PR bring supports for:
- MBTiles
- OpenStreetMap
- OpenSeeMap

![screen shot 2014-03-24 at 12 27 35](https://f.cloud.github.com/assets/655344/2498546/b912b6e8-b347-11e3-812d-89256d35de72.png)