first attempt to document core

I guess it depends on how you organize the doxygen documentation in this repo. In fact, I guess using rubydoc.info makes no sense in this particular case, since it does not use yard. I have no experience with doxygen and I don't know if is there a solution to host doxygen-generated documentation from github, similar to the yard-rubydoc.info tandem. I would say there is none, since looks like people do things like this http://rickfoosusa.blogspot.com.es/2011/10/howto-use-doxygen-with-github.html