nmbug-status: Add an OrderedDict stub for Python 2.6

Tomi Ollila and David Bremner (and presumably others) are running
Python 2.6 on their nmbug-status boxes, so it makes sense to keep
support for that version.  This commit adds a really minimal
OrderedDict stub (e.g. it doesn't handle key removal), but it gets the
job done for Page._get_threads.  Once we reach a point where Python
2.6 is no longer important (it's already out of it's security-fix
window [1]), we can pull this stub back out.

[1]: http://www.python.org/download/releases/2.6.9/