Add dstat interface req

Add requirement for dstat-interface>=1.0.2 so that it will be upgraded when the plugin is installed.

See merge request !2

Master

Hey Mike,

Just added a hook for the dstat plugin to automatically trigger install dstat-interface when the plugin is installed. This should work on Windows machines running Microdrop. Users can simply install the DStat plugin using the "File/Manage plugins../Download plugin..." as it is now hosted on our plugin repository.

-Ryan

See merge request !1

This will trigger it's installation when the plugin is installed.

Usage
=====

    python test_service.py <service bind addr> [service_time=1.]

where:

 - `service bin addr` is the ZeroMQ address to bind to.
  - _e.g., `tcp://*:6789`_
 - `service_time` is the length of time to simulate a service run in
   seconds.

Initial commit of ZeroMQ service client plugin demo.

The basic idea is to have a service exposed through a ZeroMQ interface,
responding on a `zmq.REP` socket to the following two messages:

 - `start` -> `started`
 - `notify_completion` -> `completed`

This provides a basic inter-process method for starting a service,
followed by a notification upon completion.  The plugin provides a
timeout option for each step in the DMF protocol to limit the amount of
time spent waiting for a service to complete.

__NB__ As of this commit, the MicroDrop app seems to freeze in the case
where the service times out, after the error message indicating the
protocol has failed is displayed.  This is likely due to threading
issues, which will be resolved after the plug-ins are ported to run in
separate processes.