changed input parameter from "width" to "fwhm" for Gaussian kernels
changed input parameter from "width" to "stddev" for Gaussian kernels
updated docs
Added change log entry for #2085
Updated Gaussian2DKernel example in whatsnew/0.3.rst
Merge pull request #2085 from larrybradley/gausskerns

Gaussian kernels: changed input parameter from "width" to "fwhm"