Async changes

Some changes I needed to do for async, please review them (first two commits are separate, but figured they could be useful).

You'll need to add https://github.com/xen2/ILSpy.git for ILSpy update (basically I just rebased on a newer version to have latest async changes).

Then, some Task/TaskFactory/TaskCompletionSource functions are required.
Here is my current version: https://gist.github.com/3041160 (it also includes a test for Async -- requires AsyncBridge+mscorlib).
JSIL.Async.cs is not really polished yet (which is why I didn't push it within the repository yet, I probably want check/rewrite some part).