Moved utilities to 'utils' package

This allows us to make more little utility functions
but make them more explicit by grouping them in a similar way
as the top-level nengo directory is grouped.
The top-level stays uncluttered because there's just the addition
of one folder. Seems like a good compromise!
Broke up nengo.utils.helpers

- piecewise and whitenoise go to nengo.utils.functions
- tuning_curves and sorted_neurons go to nengo.utils.ensemble
Moved tests.helpers to utils.testing