Reorganize the PNG conversion script to make it more maintable and portable

This commit rewrites the build-pngs.sh script to make a number of improvements.

Firstly it allows the script to choose a different conversion command depending
on the users environment.  It now supports either using rsvg-convert directly
or using ImageMagick convert.  rsvg-convert is used directly in preference of
convert.

Secondly it refactors the script to make the conversion a function and call that
function several times.  This is more readable and allows for easily adding
conversion of new folders with a single line change to the script.
Add support for generating PNGs in the desired size to the build-pngs.sh script

This commit adds an optional argument that may be passed to build-pngs.sh
which will cause PNGs of the specified size to be generated.  Default
size generated remains at 16x16
Minor tweak to always include size in PNG folder path
Revise the makefile and create-all-folder scripts to create and copy PNGs in various sizes

This commit heavily refactors the create-all-folder script to use functions to reduce
verbosity and make it easier to add new folders in the future.  It also copies the PNGs
in all the available sizes to the all folder.

The build-pngs target in the Makefile will now generate PNGs in 16, 32, 64 and 128 pixel
sizes
Fix up build-tar.sh to account for different size icon folders

This commit modifies the build-tar.sh script so that it copies
all the PNG subfolders which contain the different sized icons
Re-enable clean up at end of build-tar.sh script

This was disabled for testing and unintentionally left in the previous commit
Update update-README.sh script to use relative paths

This commit changes the file and image paths generated in the README to
use relative paths so that the script is not tied to a specific copy
of the repository

Also includes the latest builds of the downloads
Fix a typo in previous commit which caused bad relative paths to be generated
Fix broken relative paths again
Corrections to script so icons are not inserted multiple times into the page
Remove original PNG files

These are no longer relevant now different sized files are generated in different directories
Add a more flexible script for installing a conversion utility

This changes the install-build-svg2png-tool make target to call a new
script install-tools.sh which will check whether a suitable tool is
already available and if not try and install one.

Rather than relying only on apt-get which is platform specific it will
try and use apt-get/yum/homebrew/macports as appropriate
Add ability to use yum to install-tools.sh
Merge pull request #1 from rvesse/master

Assorted Script Enhancements

- add possibility to generate icons in different sizes 
- add parameter to scripts / commands