Async support

I modified Requestor to have async methods for all of the operations, and added async versions of all of the service methods. I started work on this before the .NET 3.5 support was merged in, so my second commit merges that in and adds "#if NET40" around all the async methods. This uses the Microsoft.Bcl.Async NuGet package so that it works on .NET 4.0 as well as .NET 4.5 and later, while the #if statements allow the 3.5 build to compile.

One thing I noticed is that the .NET 4 and .NET 3.5 builds output to the same folder -- we may want to consider putting them in platform-specific output folders.