Nodes can handle functions that don't return a value.

Fix for Issue #324
Set seed in `test_integrator`.
Added filter to direct connections where filter < dt.

Fix for Issue #319
Product network for multiplying vectors
Basal Ganglia use nnls_L2nz and output filter
Updated CircularConvolution to use Product, better kwarg error handling
Fixed some tests
Fix for test in Python3 take two
Removed leftover plot=True
Updated EnsembleArray to use slice syntax
Changed passthrough optimization to use transform_full
Added nengo.utils.testing.Timer

This is for quick time profiling of blocks of code.
We use this in a few places in unit tests.
Added tests for utils.decorators.memoize

Some of these are commented out right now due to limitations
of utils.decorators.memoize.
Improved memoize decorator

Used the existing decorator decorator to make memoize
work in more situations than before. Has the added
benefit also that introspection on memoized functions
will work as expected.
The previously commented out tests now pass.
Update utils.decorators docstring