Exclude db cache from iCloud backup, with option to include

With this change, the db cache will be excluded from iCloud backups by default when the cache is created.  A method in ````RMDatabaseCache```` called ````includeInBackup:```` can be used to include the file.  Also, a method in ````RMTileCache```` called ````includeDatabaseCachesInBackup:```` is provided to set the backup flag appropriately for all db caches contained in the tile cache.

More background information is in [#404](https://github.com/mapbox/mapbox-ios-sdk/pull/404).