Replace `path` package with `path_helpers`
Cast GStreamer state-change return values as int

Cast GStreamer state-change enumerated type return values _(i.e.,
`GstStateChangeReturn`)_ from ZeroMQ pipeline proxy methods values as integers.
Otherwise, there can be issues when calling through a proxy.

For example, when running the [MicroDrop][1] application by calling
`python -m microdrop.microdrop` from outside of the source directory, the
following error occurs when calling one of the remote API commands:

    Traceback (most recent call last):
      File ".../microdrop/gui/dmf_device_view.py", line 266, in destroy_video_proxy
        self._proxy.stop()
      File ".../pygst-utils/pygst_utils/video_pipeline/window_service_proxy.py", line 40, in __call__
        return self.sock.recv_zipped_pickle()
      File ".../pygst-utils/pygst_utils/video_pipeline/serialsocket.py", line 31, in recv_zipped_pickle
        return pickle.loads(pobj)
    AttributeError: 'NoneType' object has no attribute 'StateChangeReturn'

[1]: http://microfluidics.utoronto.ca/microdrop
Add MANIFEST.in file