Improve OpenGL ES documentation

Khronos has kindly released the docbook sources for the OpenGL ES manpages. This PR enables the binding generator to integrate these manpages in OpenTK.

Changes:
- An object model was added for documentation (`Documentation` and `DocumentationParameter`)
- `Function` instances now hold a reference to their documentation
- A new `DocProcessor` step is now invoked by the `FuncProcessor`. This can recognize docbook 4 and docbook 5 sources, and replaces the older xslt-based processor.
- All documentation processing is now performed by the `DocProcessor`. The `ISpecWriter` implementations are now solely responsible for outputting documentation in the correct format (C# docs, doxygen, etc.)

The bindings will need to be regenerated after this PR is merged.