Add option for Adaptive Histogram border pixel handling.

As discussed in #757, this adds an option to change the way border pixels are handled by `equalize_adapthist`.  The old behavior was to leave the pixels not used by the CLAHE algorithm unchanged.  We add the `mode` option to make them 'zero', or to 'crop' the image to the size that was actually equalized, with the default being the current 'ignore' behavior.

Also added a test for the new modes.
This doesn't seem to match the default `mode` of `'ignore'`. Also, it would be nice to explain these modes here, rather than in the notes. There's probably other functions in scikit-image or in scipy that have to deal with similar boundary issues. It would be nice to match their naming conventions. For example, `'zero'` seems similar to the `'constant'` mode of `ndimage.convolve`.