Fix/batch support 2nd implementation

[tenuki]

Batch support implementation (addressing: #5), allowing things like:

    server = Server('/', client)
    x = await server.batch_message(
                one=server.uno.raw(),
                two=server.dos.raw(arg="something"))

or:

    server = Server('/', client)
    x = await server.send_message(Batch(
                one=server.uno.raw(),
                two=server.dos.raw(arg="something")))