parse opengl xhtml man pages for generating brief function descriptions/comments

the functions declarations within glbinding might benefit from the brief description, parameters, and return section of the opengl man pages. however, this needs to be applied in a way that, e.g., `glVertex###` variations all use the same slightly adapted `glVertex` description.

some minor drawbacks to discuss here:
- the parsing even though the pages are xhtml, might not be that easy,
- parsing might take much longer and man pages should be loaded at generation time or stored within the glbinding repository as well?,
- the deployment package, mainly the functions header massively increase in file size (which should not matter that much, since only comments that are discarded efficiently by compiler?).