Minor cleanup in nengo.utils
Add test for nengo.utils.tuning_curves().
Add tuning_curves_along_pref_direction().
Implement meshgrid_nd function.

Numpy's meshgrid did not support more than two dimensions
or ij-indexing before version 1.7.
Refactor tuning_curves functions, add tests

- Renamed tuning_curves_along_pref_direction to response_curves
- Made tuning_curves and response_curves more robust
- Added tests for LIF neurons (so far only direct mode neurons
  were tested).
- Easier plotting of tuning curves with dim >= 2 in tests.
Respect radius in tuning curve calculation.
Change eval_points shape in tuning_curves

This makes it possible to unpack the result of `tuning_curves`
to plt.plot in the 1D case. The 2D case is only a little less clean.
Added tuning_curves notebook

Shows the common use cases of tuning_curves and response_curves.