Write MD5 footer to `pip freeze` output

This is an experiment with writing the MD5 for the requirements to the bottom of the `pip freeze` output.

The idea is that the MD5 could be used to quickly compare whether two virtualenvs have the exact same set of packages.

Sure, on a UNIX system one could pipe `pip freeze` to `md5sum`, but this is not necessarily easy on a Windows system and even with the pipe, you'd have to run `pip freeze` twice if you wanted to see the package list and the MD5.

I'm not convinced that this is worth it; I'm actually learning towards thinking it's not worth it, but since it took some time and it might be useful to someone, I thought I'd share it, in case it was useful to someone or sparked some discussion about comparing pip requirements.

Cc: @aconrad