Gabor filter docstrings

Added examples for both ``gabor_kernel`` and ``gabor_filter``. Moved ``n_stds`` variable in both functions to the parameter list.
I find the description of the function to be very short. Would it be possible to add one or two sentences saying what a Gabor filter is (as in the wikipedia page: it's a Gaussian Kernel function modulated by a sinusoidal plane wave).