THRIFT-2456 THttpClient fails when using async support outside Silverlight

THRIFT-2456 THttpClient fails when using async support outside Silverlight
Client: C#
Patch: Adam Connelly & Jens Geyer

This closes #97

commit 080b28eac31516ef9e3f988a87d890769f0751cd
 Author: Adam Connelly <adam@resdiary.com>
 Date: 2014-04-09T12:46:27Z

THRIFT-2456 Support async operations outside Silverlight

I altered TTransport to throw the NotSupportedExceptions to make it more obvious what's going on if someone generates their client using the async option, and then uses a transport that doesn't support it.