Corrections to ethernet IO.
Neaten the simulator.

 - Raise a NotImplementedException if `run` is called twice without an
   intervening `reset`.
 - Move a lot of the PACMAN code out of `run` into a private method.
 - Provide the `reset` method
 - Add a lot more comments
Add more documentation.
Merge branch 'cleanup-on-fail' into neaten-simulator

Conflicts:
	nengo_spinnaker/simulator.py
Make it clearer which dt affects the host.
Merge branch 'master' into neaten-simulator

Conflicts:
	nengo_spinnaker/simulator.py
Address merge comments.
Fixes for changes in based PR.
Merge pull request #83 from ctn-waterloo/neaten-simulator

Neaten Simulator, Ethernet, add some docs