Adding HTTP Message initializers to OHHTTPStubsResponse

I.e.
+[OHHTTPStubsResponse responseWithHTTPMessageData:
                                     responseTime:]
+[OHHTTPStubsResponse responseNamed:
                         fromBundle:
                       responseTime:]
Merge pull request #27 from nxtbgthng/http-message-response

Adding HTTP Message initializers to OHHTTPStubsResponse