Simplifying ServerSentEventsTransport
 - NotSafeForWork - Removing unnecessary SafeInvoker from ServerSentEventsTransport
 - DisposingOfDisposer - unifying request cancellation
FoundABetterHome - moving ProcessResponse to ClientTransportBase class
HowToDismantleALongPollingBomb
 - merging LongPolingTransport and PollingRequestHandler
 - removing NegotiateInitializer
 - tests and clean up
PortableDroid - updating references to Microsoft.Net.Http NuGet package

Updating references to Microsoft.Net.Http NuGet package to fix build errors appearing when building a Xamarin Android project after adding the Signalr Client NuGet package to the project.