Changed installer so people can chance overtone to ther eown fork of the emacs live project

Hi,
I love emacs live and have my own personal pack etc that I wanted to use.
I therefore have submitted that to my fork and changed the installer to use a generic variable which can be set at the top of the shell file.

This means that I can pull my latest code with the pack directly.

Please note I tried to only change the installer but my other contribs to my own dir have been added.  I'm not sure how to remove them from the pull request.

If you can let me know and want a pull request with the one change then I would be glad too.

Many thanks