Removed the code which performs the operations on another thread

Removed the code which performs the operations on another thread and gracefully handled it without introducing any breaking changes. Also, marked the methods as [Obsolete](http://msdn.microsoft.com/en-us/library/system.obsoleteattribute(v=vs.110).aspx) as they should be removed on the next major version release of the library because they cause confusion.

If anybody whats to perform the Serialize, Deserialize and Populate operations on a background thread in any client applications (windows phone app, windows store app, WPF app, etc.), he should wrap the code inside `Task.Run` by himself. This isn't job of the library.

Refer to "[Should I expose asynchronous wrappers for synchronous methods?](http://blogs.msdn.com/b/pfxteam/archive/2012/03/24/10287244.aspx)" article by Stephen Toub for more information.