gce - instances api

Small note: Instance.class is used as both input as output (mirroring gce itself) this even though for input 4 fields (status, selfLink, image and id) are not supposed to be set and will be set in output. 
As the user facing object is always the output version (the input object is used just for serialization) where those fields are set this avoids having to create extra parsing code and an extra version of Instance just for the sake of not having those 4 fields for input. Hopefully the reviewer will agree :)