jsonclass: Replace "type() is" by isinstance()

Allows to use custom dictionaries, ...
New methods: dump(), load(), Fault.dump()

- dump() returns the JSON-RPC dictionary instead of its string representation
- dumps() calls dump()
- same thing with load() and loads()
- Fault.dump() is the dictionary equivalent of response()
New method: SimpleJSONRPCDispatcher._unmarshaled_dispatch

- Takes a loaded dictionary as a parameter (jsonrpclib.load/s)
- Returns a JSON-RPC dictionary (jsonrpclib.dump)
- _marshaled_dispatch wraps _unmarshaled_dispatch (keeps its behaviour)