libplist: Add HEAD, add Python bindings, remove libxml2 dependency.

* Added HEAD
* Removed unnecessary CMakeLists.txt patch (now done via options)
* Added --python option for Cython Python bindings
* Added Caveat
* Added fix for CMake from opencv.rb
* Removed libxml2 dependency, provided by OS X

Related to #18396

Hope the `libxml2` part is ok. I read [here](https://github.com/mxcl/homebrew/wiki/Acceptable-Formulae) that it's ok to use `depends_on` in this case, but also [here](https://github.com/mxcl/homebrew/wiki/Formula-Cookbook) that homebrew tries not to duplicate dependencies.