Download cancellation and progress reporting

[shana]

• Added the ability to cancel downloads and progress reporting.
• Fixed a typo
• Made sure all tasks have the cancellation token set (so we can cancel at will)
• Fixed calling the Finally in-thread handler - it wasn't getting called in all situations, which made things hang sometimes (namely when calling StartAwait).
• Added a local http server to serve files for the download tests, so we don't have to rely on the network for such things and we can control exactly what comes down the pipe.
• Renamed tests to explain what they're testing
• Fixed logging of exceptions to log the inner exceptions as well, so we can see what got thrown where
  • As part of this, moved all GitHub.Logging classes to their own namespace GitHub.Unity.Logs so that it's easy to filter them out when outputting caller stacktraces
• Exposed the web request timeout value as a property in ApplicationConfiguration (renamed from AppConfiguration for clarity), so we can tweak it during testing.
• Exposed the web request timeout in the UI so users can tweak it (they're DNS or network might be very slow at resolving things)
• Added a custom Progress class
  • The built-in System.IProgress<> interface is suited for a push model where the consumer sends in a IProgress object to be notified of changes (which might be useful for global progress notification), which doesn't suit our chaining callback system very well. The implementation in Rackspace.Threading does a Post to the main thread, so we really shouldn't be using it in our threaded model.