Changed RpcProxyGenerator to correctly erase params and return types (refs ERRAI-148)

This makes our proxy generator generate correctly-erased method signatures for all known
scenarios except type variables with multiple upper bounds (like <T extends List & Serializable>)

HOWEVER, there's a bug in the GWT compiler that casuses it to reject our erased method
signature when the type variable itself is used as a parameter type (see the commented-out
method in TestRPCService).

Bottom line: this change makes more stuff work, but still not everything required to
resolve ERRAI-148.