Add support to ignore sections conditionally based on a Python expression.

@jimfulton : here is a cleaned up version, addressing all the comments you made before.
All tests are passing, see https://travis-ci.org/pombredanne/buildout

Section titles can now have this form:
 [sectionname:Python expression] # optional comment

If the Python expression evals to False, the section will be ignored.

Expressions have some defaults to support common conditions such as:
 [sectionname: not windows] # ignore this section on windows

Section title lines in the traditional form are still supported of
course:
 [sectionname] # optional comment 
or
 [sectionname] ; optional comment