Start Documentation of the Language Features

With this Pull Request I propose a language documentation inside of the ```doc``` folder. I found it very helpful to have an overview of the already existing methods and the locations in the source code. So I would like to submit my currently private 'notes' of the source code to the repository. Maybe other people found it helpful. At the moment I only created an overview of the core Classes and Modules defined in C. I would plan to extend the documentation also to the Ruby code and the built-in GEMs.

Pull Request Contains:
* new ```doc/language``` folder with language documentation
* additional ISO comments in the C source code
* ```mrbdoc``` tool to re-generate the documentation