Added instructions to tap homebrew/science for OpenCV

Homebrew removed OpenCV from its main repo on Sunday. This ensures you
still can install it.

Details: https://github.com/mxcl/homebrew/commit/9495b06ef3f35be8711902ce268
Merge pull request #26 from myersjustinc/tap-opencv

OpenCV was removed from Homebrew core this past weekend. Fix instructions.