Initial python version which extracts from an mbox

(the perl module for parsing mbox was hanging on the input I gave it.
sadness)

Differences:
* no progress bar (sadness)
* didn't implement the commit-only-after-x-transactions
* uses the X-Bugzilla-Type header to detect the email type and decide if
  we want to save it or not
* because it does that, I think I can go ahead and assume that the
  equivalent of $in_bug is always true :)