Run NSURLConnection delegate on background thread

I modified the MSClientConnection class to configure an
NSOperationQueue to execute the delegate methods for the
NSURLConnection class when executing a request. The NSURLConnection
class by default will execute the delegate on the thread that started
an asynchronous load operation. By executing the delegate methods using
an NSOperationQueue, the calling thread can simulate a synchronous
operation or block while waiting for table operations to complete. This
is important when trying to integrate the Azure Mobile Services SDK
with Core Data using an NSIncrementalStore class.