FACES

@drsnyder 
Creating this PR so it's recorded somewhere, but will probably delete this branch.

Face detection was pretty straight forward, the hardest part was building opencv. I used [this tutorial](http://docs.opencv.org/2.4.4-beta/doc/tutorials/introduction/desktop_java/java_dev_intro.html) to build opencv on my laptop and [this tutorial](http://docs.opencv.org/doc/tutorials/introduction/clojure_dev_intro/clojure_dev_intro.html) to bake it into clojure using Leiningen.

If we wanted to get this into production, we'd have to figure out how we'd include opencv. Maybe we can build it once and then store it in our Archiva repo. We'd also have to clean up some of the code in the `vignette.thumb-modes.face-detection` namespace, since the hack here is to first create a new thumbnail containing the face, then thumbnail using the original params after that face thumbnail is created. Works, but maybe there's a better way to do that. Would probably also want to pad around the face, maybe?