Create Brewfile

I recently learned that homebrew can check for a file called `Brewfile`, and execute its contents with homebrew. It appears to be undocumented, except by [this blogpost](http://robots.thoughtbot.com/brewfile-a-gemfile-but-for-homebrew) by thoughtbot. Still, it looks nifty, and might make it easier to get new Mac developers up and running.

Also switched from mysql to [mariadb](https://mariadb.org/), because Oracle kills open source projects.