Implemented UDP socket utility function for Nodes

Initial working prototype of udp socket functions for nengo Nodes. 
- Sockets can be easily inserted into nengo models by creating the socket class with appropriate parameters, and then creating a nengo.Node and passing the socket class' run function as the output parameter.
- Socket class is designed to be not require any special tweaking on the simulator side, and also designed to close itself after a predefined period of inactivity (or by the user if desired).

I think as a result of the recently committed #518 changes to the tests so that Travis works again, that you need to change your tests to use the new "plt" functionality.