allow comments in multi-line options

[Georges Racinet]

We have several multi-lines options, the most prominent one being 'addons'
Only one sports a (limited) form of inline comment : revisions

Many people get bitten by this, including myself while preparing the talk about the recipe for opendays, and this ended up as a demo error because of forgotten vcs update.
In short words : annoying and potentially dangerous .

We'll need a utility function to do it once and for all and to replace all the 'splitlines()' with it.
That utility function should follow the official format : http://docs.python.org/2/library/configparser.html

For the record, buildout configuration format is not exactly INI (because of the '+=' syntax)