Although very unlikely here, it's good pratice to write code without possible
race conditions. The only remaining one here is when anouther process create
a file (with the same output path) between its deletion and its creation.

They should not be needed when buildout handles its own serialisation.

Based largely on work by Timothée Lacroix <timothee.lacroix@tiolive.com>,
plus:
- reworked loader classes
- reduced unnecessary diffs
- added rawfile and rawfolder types
- documented & tested

umask cannot set a regular file as executable, nor allows setting
suid/sgid/sticky bits.
Using chmod properly, there is no security problem.

Useful when extending another section from same file.

This test case was accidentally reusing a previous template.

Noticed by Timothée Lacroix <timothee.lacroix@tiolive.com>

Patch by Timothée Lacroix <timothee.lacroix@tiolive.com>

Patch by Timothée Lacroix <timothee.lacroix@tiolive.com>

Reported by Timothee Lacroix.

Loads support should not be required: the use-case is writing python values
when generating a buildout.cfg file.

Test the latter, and document how to work around the former.

Also, fix pyc files gitignore globing.

Buildout way couldn't install some (dependency) eggs for an unknown reason
(zope.exception on some python environments, Jinja2 needed by a future
extension...) and "setup.py test" just works, with fewer lines at this.
Also, drop zope.testing dependency altogether:
- zope.testing.doctest is deprecated
- zope.testing.renormalizing is not really needed for this test