Update _client.py to allow server and port in the Client constructor

[adam-parker1]

Simplify python client instance creation

pyuda.Client.server = 'my.server.hostname'
pyuda.Client.port = 56789
client = pyuda.Client()

Can now we replaced by

client = pyuda.Client('my.server.hostname', 56789)

[adam-parker1]

Also, I think __metaclass__ = ClientMeta is redundant in _client.py if we are using six.with_metaclass(..)

[adam-parker1]

If we are not targeting Python2 anymore, maybe we can replace the six.with_metaclass(....) with the standard metaclass(...) and remove the six dependency?

[adam-parker1]

Review for UDA v3.0