bitarray is an optional extra.

Without bitarray, utils.probes.SpikeProbe is None, this is caught by the
builder and an error is raised if the user tries to make use of this
functionality.
Merge branch 'master' into bitarray

Conflicts:
	nengo_spinnaker/builder.py
Merge pull request #45 from ctn-waterloo/bitarray

Make `bitarray` an optional extra