Refactor middleware a bit into separate methods
Change TRACK_IGNORE_URLS to give tracking a blind eye

- if the developer specifies `TRACK_IGNORE_URLS = ('^api/', '^admin/')`,
  for example, no visitor object will be created/updated despite the
  session being updated.
Update README and CHANGELOG to reflect change of TRACK_IGNORE_URLS
Of course, python 2.6 doesn't have `.total_seconds()`

- Implement the equivalent as a utils function
Merge pull request #46 from mjschultz/ignore-urls-45

Increase scope of TRACK_IGNORE_URLS