Fix for accented author names being omitted from stats.

Accented author names weren't being found due to the regex searching with \w instead of [:alpha:], which is accent-safe, so I changed that.

However, for that regex to work the encoding of the string (returned from `git log --stat`) needed to be enforced. As per [this SO answer](http://stackoverflow.com/a/13194139):

>Ruby 1.9 and 2.0 will use composed UTF-8 strings if you use UTF-8 encoding, but will not modify strings received from the OS. Mac OS X uses decomposed strings (two bytes for many common accents like é in UTF-8, which are combined for display). So file system methods will often return unexpected string formats, which are strictly UTF-8, but a decomposed form.

>In order to work around this, you need to decompose them by converting from the 'UTF8-MAC' encoding to UTF-8