Merge pull request #21 from RoProducts/mbtiles

Mbtiles from local database
Prevent NPE on missing layers
Fix not Sqlite Layers not drawing
Fix typo on class name
work 080914 zooming mb tiles, not complete yet
zoom interpolation limited to one zoom level, javadoc added
Merge branch 'mbtiles' of https://github.com/RoProducts/MapStoreMobile into mbtiles