Fix kernel-size calculation for non-zero theta.
Add `bandwidth` parameter to `gabor_kernel`

Note that this changes the API of `gabor_kernel` and `gabor_filter`: The input parameters are rearranged and positional arguments are changed to keyword args.
DOC: Tweak docstrings for Gabor filter
Update gabor example.

The parameter order to `gabor_filter` changed so this example was broken. Also, add plots of the Gabor responses to the demo.
Merge pull request #2 from tonysyu/gabor

Change API for Gabor filter