use month numbers in urls

Hello,

There is a problem with locale specific month names in urls:

* in english locale - http://example.com/gallery/2014/aug/
* in russian locale - http://example.com/gallery/2014/авг/
* the same after urlencode - http://example.com/gallery/2014/%D0%B0%D0%B2%D0%B3/
* regex pattern in urls.py - `(?P<month>[a-z]{3})` - it doesn't match last url

I think it would be better to use month as number in urls. Here is the patch for this issue.