Gabor filter docstrings

Added examples for both ``gabor_kernel`` and ``gabor_filter``. Moved ``n_stds`` variable in both functions to the parameter list.
It's true that shouldn't be a problem for Travis-CI, but a *user* running `nosetests --with-doctest` should experience problems. I say *should* because `nosetests` doesn't discover `_gabor.py` because of the leading underscore.

I think I've advocated this before but there was a counterargument that I don't remember: If there's an example that we don't intend to run as a doctest, we can just write it as a literal-block.