Add TileProvider that stores images on disk

[johnpryan]

Some users have asked for better image caching that can store tiles on disk #276

[josxha]

The policy of OpenStreetMap requires caching of tiles.

Cache Tile downloads locally according to HTTP Expiry Header, alternatively a minimum of 7 days.

https://wiki.openstreetmap.org/wiki/DE:Tile_usage_policy

For this reason I'm really looking forward to this feature.

[Lootwig]

Would it make sense to use the same technology to also implement preloading of a specific section (like google's offline maps - so, all zoom levels for the section at once rather than on-demand)?

[mootw]

@johnpryan This issue should be closed. Fixed by #841