Adds support for ImmutableMap in jclouds gson

Specifically making sure deserialization works properly.
> I am not completely convinced about the second point, but it seems it will be somewhat confusing either way anyways.

My take would be that the class itself serves as a type adapter which does the adapting by deserializing JSON. So the latter is an "implementation detail" whilst the former describes the purpose of the class. But I'm also OK with `ImmutableMapDeserializer` if you really prefer that.

Irrespective of that, how do you feel about moving the argument until after all the TAFs?