new command: resize selected items by aspect ratio

Hi Ale!

Here my first contribution to Orange Commands :)

I needed to create a icon-font with [Font Custom](http://fontcustom.com/) so I wrote it to resize a bunch of icons preserving aspect ratio, and then export each icon to a SVG file with the [Export SVG command](http://fireworks.abeall.com/extensions/commands/Export/) modified.