Use ReaderWriterLockSlim

Improve performance of FileCache by using ReaderWriterLockSlim to get
more concurrency in reading from cache.
Merge pull request #15 from josh-cooley/FileCache-ReaderWriterLock

File cache reader writer lock
Adding GetUri to HttpTileSource
Merge pull request #5 from BruTile/master

Merge updates from origin
Merge remote-tracking branch 'origin/master' into tile-provider-async

Conflicts:
	BruTile/Web/HttpTileSource.cs
	BruTile/Web/WebTileProvider.cs