Allow a magic field in a class to specify the default type adapter

[GoogleCodeExporter]

I want to specify the JSON serialization/deserialization semantics within my 
class itself, through a static field. 

class A {
  private static final TypeAdapter<A> GSON_TYPE_ADAPTER = .....
}

The big value is that all users of my class get automatic benefit of type 
adapter, they don't need to remember to register it.

Original issue reported on code.google.com by inder123 on 8 Mar 2014 at 8:17