fix image URL
Add mobile-specific InternalTo signatures.

After this change, the build will fail without dropping SIGNED condition on *unsingned* builds. To build Rx assemblies from sources outside Xamarin, please do so.
Add -d:HAS_AWAIT for await support in XA.

This is limited to assemblies that are supported in XA.
remoting is not going to be supported in XA so remove it.
added Makefile for building and packaging everything.

Also remove prebuilt dlls now that mcs doesn't generate broken dlls and
we don't have to resort to csc anymore.