Fix URLs directly related to Buildout

This includes URLs to bootstrap.py, the Buildout own code repository, the bug tracker, and some Zope related components which have moved to Git and Github in the meanwhile.

This also fixes the bug reported on Launchpad at https://bugs.launchpad.net/zc.buildout/+bug/1279770

(On a side note, quite a few bugs have been opened on Launchpad, and comparing the answer time between these and the one opened on Github, I guessed the Github issue tracker was the canonical one, and the Launchpad tracker should probably be closed for new tickets...)