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.
Merged wp81 into portable-net45+sl5+netcore45+wp8. Before it was seperated because it was using a different version of Microsoft.Net.Http but since the version is the same now it does not have to be separate.