OpenNI2 support and Mavericks HEAD requirement fix

Dear @fran6co, I've add a simple modification to your formula that enables the use of an experimental (though stable enough) PCL repo with OpenNI v2 support. This repo is currently pull-requested and until the PCL project accepts it, it'd be nice to be able to use with the appropriate formula optional parameters. I've tested it with my own ASUS Xtion Pro Live and works like charm. 

I've also modified the Marvericks/HEAD warning message because the warning did not happen until the install phase. It now warns the user before any action (patch, install) takes place and stops formula install if --HEAD is not specified on Mavericks and/or with OpenNI2.

Hope you find my contributions useful, thanks a lot.