KEGG KGML parser and graphical visualisation from Leighton

This pull request is mainly as a discussion point for Leighton's (@widdowquinn) KEGG KGML parser and visualisation work, originally on the following pull request, but the branch was a little messed up: https://github.com/biopython/biopython/pull/155

Original mailing list thread: http://lists.open-bio.org/pipermail/biopython-dev/2013-February/010326.html

Leighton's blog posts about this work:
http://armchairbiology.blogspot.co.uk/2013/01/keggwatch-part-i.html
http://armchairbiology.blogspot.co.uk/2013/02/keggwatch-part-ii.html
http://armchairbiology.blogspot.co.uk/2013/02/keggwatch-part-iii.html

I shall add some comments in a moment...