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 pull request #6 from josh-cooley/master

Merge updates from origin