Added support for OpenNI2, cleaned Mavericks/HEAD issue

Added support for PCL with OpenNI2 currently on pull request from https://github.com/kwaegel/pcl. Works wonderfully. This is a trick until PCL guys merge it (if they ever do it).

I have also cleaned up a little bit the Mavericks/HEAD warning and detection.

Steps:
# tap openni2 repo form totakke
brew tap totakke/homebrew-openni2
# install pcl with the --with-openni2 option
brew install pcl --with-openni2 --HEAD