Use mailmap in github stats

In the 1.4.0 release notes I manages to get my name in using 3 different variations http://matplotlib.org/users/github_stats.html. I thought that looked rather silly so this is an attempt to deduplicate this.

IPython deduplicates various spellings by using the .mailmap file so I have just copied this to our repository and added the relevant variations of my name to the mailmap

I think that at least @pelson  and @pwuertz  also appear more than once in the log so we might want to add them. This could be back ported to the 1.4.0  documentation branch.