Add Doxygen documentation

[ventsyv]

I think the code will benefit from adding doxygen. I'm familiar with most of it, so I can add as much as I can and you can add the rest. What do you think?

[jflesch]

Please do, of course :)

[ventsyv]

I realized you are using docstrings, so it's just a matter of generating the documentation, no need for doxygen tags.