Gabor filter docstrings

Added examples for both ``gabor_kernel`` and ``gabor_filter``. Moved ``n_stds`` variable in both functions to the parameter list.
I think the examples are run as doctests, if I'm not mistaken, so plotting may cause problems on non-headless displays.  Then again, is that a problem, since we've configured Travis-CI to be headless, and since mpl is a dependency?