Add ICO files

This sequence of commits adds scripts for generating ICO files from the PNGs, it also expands the range of sizes of PNGs in order to generate good ICO files that should support a wide range of resolutions.

ICO files are generated by a script from the PNG files in the repository.  These are now included in the downloads.

I also improved how the README is generated to make it easier to edit the text outside of the script.  The README now also covers how to generate PNGs and ICOs.

As usual you likely need to regenerate the PNGs once this is pulled in since SVG-PNG conversion is really borked on my machine, sorry!

Cheers,

Rob