Assorted Script Enhancements

First off thanks for putting this awesome project up there, this pull request contains a series of commits with enhancements to the scripts based on my playing around with this today:

- build-pngs.sh can now use rsvg-convert or convert (ImageMagick) dependent on what the users environment has available
- build-pngs.sh takes optional argument specifying the size of the PNGs to generate, default remains at 16
- build-pngs make target generates PNGs in sizes 16x16, 32x32, 64x64, 128x128
- create-all-folder.sh now copies each size of PNG
- build-tar.sh packages up all the different sized PNGs
- update-README.sh uses relative paths for links so they are not tied to a specific clone of the repo, also now embeds the 32x32 size PNGs instead

Btw I notice that when scaled up some of the PNGs are slightly cropped, I'm not sure if this is a case of a bad conversion from SVG to PNG or an artifact of the original SVGs.  I don't have an SVG editor to hand so I haven't been able to confirm what the issue is, you should be able to see this by looking at the front page of my clone.