Fix calling (raw_)rpc wrapped methods

Make it possible to call methods wrapped with @rpc and @raw_rpc as
any normal methods, by keeping the normal behaviour of Python methods
and any RPC call must be by invoking method.rpc_call(channel, ...).
I don't get this comment sorry. 

The abstract class (interface "a la" Python) is as useful as any interface in OOP language i.e. "Program to an interface, not an implementation", that's why it's always good to define interfaces (I always try to do and there is more patches coming that do exactly that :)), and to expose them for extendibility, instead of implementation (unless they are meant to be used as such i.e. for instantiation), and that's why I like to define interfaces alone (separated from the implementation) which also go pretty well with Python duck typing paradigm.