Count spikes from the retina which fall within a given region.

Todo:
 - Ensure region is represented correctly
 - Match Terry's API for this
Correction to match protocol spec.
Different retina packet types.
Revert "Different retina packet types."

This reverts commit ea25006f903f6b1860d1c602884094b9d2f5dc90.
Merge branch 'master' into spinn-countspikes

Conflicts:
	nengo_pushbot/spinnaker.py
Merge pull request #7 from ctn-waterloo/spinn-countspikes

`Countspikes` on SpiNNaker