Update to .NET 4
Update to .NET 4.5
Converts core LoveSeat methods to async / await semantics.

Methods have been renamed to follow MS guidelines, e.g. GetDocument()
becomes GetDocumentAsync().

Return async in all methods where we trigger an http request.
Using JSON.NET from NuGet
Renames LoveSeat projects to LoveSeat.Async.
Updates readme
Adjusts namespace to LoveSeat.Async