Adds support for ImmutableMap in jclouds gson

Specifically making sure deserialization works properly.
The adapter needs to return the right type. As the adapters are implemented as final classes, I couldn't just extend it
It might be possible to dry up, but it would require more refactoring.
I had some issues parametrizing it, so I didn't do it. However, it might be possible to have the Map adapter extend the ImmutableMap one and then reuse most of the functionality. I will give it a try tonight or tomorrow.