Let's actually make this work with NET40

I know there was a good reason why I didn't publish a NET40 version of this assembly (AsyncTargetingPack? TaskEx?) so here it is:


 - [x] still using PCL, but publish a net40 package with the 1.0.0 release
 - [x] replace AsyncTargetingPack with Microsoft.Bcl.Async
 - [x] update xUnit and test runner