Minor improvements and updates

Exposed AuthKey as public property (in addition to PassKey) rather than
having to call ActionGetIndex()
Added GenerateTorrentDownloadUri() method
Updated JSON API Reference URL
Improved Dispose()
Added Api.GetCollage()
Added ActionCollage model
Added ApiTests.GetCollageTest()
Added Random.GetCollageId()
Added Api.SessionIsValid()
Added ApiTests.GetSessionIsValidTest()
Various minor improvements