add ico save support

Add ico save support (png)

* all images are stored as png
* params: sizes, e.g. [(16, 16), (24, 24)]

You might want to check the quality of multiple resizes down from the initial icon size vs a single resize from the original for each size. (Note that there have been some quality fixes to resizing lately, so quality may be better now than it was a few days ago)