Server-side native object support

[wemoloh]

Passing a native object from the client to the server as a method argument works as documented, but server-created native objects can't be passed back:

import eider

class MyRoot(eider.LocalRoot):
    def foo(self):
        return MyClass()

class MyClass:
    def bar(self):
        return 'baz'

eider.serve(12345, root=MyRoot)

>>> from eider import BlockingConnection
>>> conn = BlockingConnection('ws://localhost:12345/')
>>> sess = conn.create_session()
>>> b = sess.root().foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "eider.py", line 1267, in __call__
    return self.robj._rsession.conn.loop.run_until_complete(super().__call__(*params))
  File "/usr/local/lib/python3.4/asyncio/base_events.py", line 373, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.4/asyncio/futures.py", line 274, in result
    raise self._exception
  File "eider.py", line 1006, in dispatch
    result = self.getresult(rcodec, rcall.rsession, msg)
  File "eider.py", line 1066, in getresult
    return rsession.unmarshal_all(rcodec, msg['result'])
  File "eider.py", line 222, in unmarshal_all
    return self.unmarshal_all_inband(obj, srcid)
  File "eider.py", line 228, in unmarshal_all_inband
    return self.unmarshal(obj, srcid)
  File "eider.py", line 244, in unmarshal
    obj = self.unmarshal_obj(ref, srcid)
  File "eider.py", line 685, in unmarshal_obj
    raise LookupError('Unknown session: {}'.format(rsid))
LookupError: Unknown session: -1