Portable versions of NativeMessageHandler and NativeCookieHandler

[ncruces]

This is a first try at fixing #188. Adapted from #175.

NativeCookieHandler mostly works, though API mismatches forces us to jump through some hoops.
I based this off #187, which I think is a good idea?

NativeMessageHandler still has some gaps:

• throwOnCaptiveNetwork works;
• customSSLVerification doesn't seem to require anything special;
• DisableCaching would require inheriting from WebRequestHandler which isn't portable (full .NET only);
• RegisterForProgress is yet to be done.

Any feedback would be greatly appreciated.

[ncruces]

Any interest in me further pursuing this?

NativeCookieHandler, which is what I needed, is done.
RegisterForProgress, I could do, if requested.
DisableCaching requires the approach rejected in #175.

[CliffCawley]

I'd like to see this officially supported. Right now I can't just drop in NativeMessageHandler instead of HttpClientHandler because the CookieContainer contents are ignored by NativeMessageHandler and I need to send cookies with my request. (Using with Xamarin iOS/Android)

[jamesqo]

+1, I think this is a great idea as well. This would allow the handlers to be used reliably in PCLs without any fear of breaking for Windows clients, while providing the speed benefits when possible on iOS/Android.