Add docstring to top of lib/extract.py file

[seanh]

I think it couldn't hurt for this module to have a docstring at the top of the file explaining what the module is for. I know from setup.py that it's there to provide the extract_ckan() string extractor function for Babel, but I think looking at the file on its own it's not so obvious.

Also a couple of other small fixes:

jinja2_cleaner looks like a helper function only meant to be used by extract_ckan(), might make things clearer if it was called _jinja2_cleaner(), just so it doesn't look like a public function the module wants to export.

jinja_extensions seems to be a module-level variable that is only used in one function, could be moved into the function, unless you think there might be more functions that want it in future.