add .mailmap

Some people have committed with multiple settings for `user.name` and/or `user.email`.

`.mailmap` allows `git shortlog` to collapse entries.

E.g. `git shortlog -sne`

Before:
![image](https://cloud.githubusercontent.com/assets/677704/2941261/952e0d5e-d99e-11e3-9b1b-3c4be7907e7e.png)
After:
![image](https://cloud.githubusercontent.com/assets/677704/2941264/9fb47ab0-d99e-11e3-999b-21eb5d2f0d96.png)