Fix for connection pruning warnings.
Corrections to __init__
Get split edges correctly.
Add reference constraints.
Initial work on a network analyser.

Usage is similar to the Simulator:

>>> with nengo.Network('Sample') as model:
>>>     # Build model..
>>>
>>> analyser = nengo_spinnaker.Analyser(model)
>>> analyser.create_hypergraph_as_json('my_model.json')
Merge branch 'development-analyser' into development