Add support to ignore sections conditionally to a Python expression.

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
Merge remote-tracking branch 'buildout/master' into header_expressions
Use more explicit variable names
Merge remote-tracking branch 'buildout/master' into header_expressions
Now supporting semicolon and hash comments.
Merge pull request #76 from pombredanne/header_expressions

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