ENH: ndimage: gaussian filter truncation

This is an extension of https://github.com/scipy/scipy/pull/239
I added a commit that strengthens the existing test, and adds tests for more of the affected functions.

There are two API enhancements in this PR:
* Add the `truncate` argument to `gaussian_filter` and `gaussian_filter1d`.  This controls the size of the filter, measured in terms of the standard deviations of the gaussian.
* Add `**kwargs` to the signature of `gaussian_gradient_magnitude` and `gaussian_laplace`.  Any additional keyword arguments passed to these functions are passed on to `gaussian_filter`.  This allows these functions to use the `truncate` argument.

This API looks reasonable to me.  If there are no objections, I'd like to get this into 0.13.