Added map2slim functionality using the goatools framework.

Hey,

I implemented the GO Slim mapping into the goatools framework.
Some information about GO Slims: http://www.geneontology.org/GO.slims.shtml

This implements the same algorithm as the go-perl map2slim.pl script described here: http://search.cpan.org/~cmungall/go-perl/scripts/map2slim

This also includes a few fixes (mainly code style though), of the original code.
If you want, you can merge it into the main repo. I would be happy to have contributed :)

Cheers!