Made TileJsonTileLayer fetch Async

We found that MapBox was causing significant lag in our app caused by
getBrandedJSON() which subverted the NetworkOnMainThreadException by
starting an Asynctask and waiting for it to finish, up to ten seconds.
This commit fully moves the fetching to a separate thread.
Merge pull request #449 from damianw/async-branded-json

Made TileJsonTileLayer fetch Async