Add python3 md5 import

[cam-stitt]

This should allow expiringdict to have python3 support. I ran the tests and everything passed.

[chrismcguire]

+1. The existing import issue breaks the installation of flanker in python 3.3.

[ierror]

+1. I'd like to use flanker with python 3.4

[svisser]

This was resolved two months ago in this commit: fbddf2a

So it should be a matter of packaging that commit and releasing to PyPI (see also: #9).