Added noise to least square decoder solver

- Fixes the failing test_product
- Also added a helper to plot tuning curves,
  needs to be refactored a bit though
Simplified connections with simulator ops

- Created a new ProdUpdate operation
- Made DecodedConnection and DecodedNeuronConnection subclasses of
  SignalConnection, and renamed DecodedConnection to
  NonlinearityConnection
added integrator example
added a spike probe

call this with, e.g. model.probe('A.spikes')
then you have data=sim.data('A.spikes')
updated the many neurons notebook
standardizing notebooks and notebook naming
Updated spike probe with new connection types
Fixing typo in helpers.py
Add white noise signal generator

- Small connection change as well
Converted addition example to ipython
Converted multiplication example to ipython notebook
Addition example converted to ipython notebook
Multiplication example converted to ipython notebook
Squaring example converted to ipython notebook
Merge branch 'master' into chris-examples

Conflicts:
	examples/controlledintegrator.ipynb