Add requirements.txt, modify docs conf accordingly.

Pull in Nengo using Git, modify when a Pip release is made.
Correction to requirements.txt
Update requirements.txt
Update requirements.txt
Update requirements.txt

Include bitarray.
flake8 and documentation for simulator.
Further documentation on Nodes.
Include images in doc.
Bugfix to decoder equivalency check.
Fixes to SpiNNlinkUSB, FilterVertex.
Filters default to zero if the time constant is None.
Prune examples, modify to fit new API.
Changes to track Nengo.

Minimum changes to EnsembleVertex and Builder to get things to run.
Remove unused connection type.
Should make config work
Correct default decoder
Merge pull request #46 from ctn-waterloo/nengo-temp

Nengo Tracking
Merge branch 'master' into bitarray

Conflicts:
	nengo_spinnaker/builder.py