add html documentation to repository

I've added two files: doc.md and make_doc.py to the repository. I find it easier to edit the documentation in markdown and then generate the HTML, but we can change it if it's too convoluted. To generate the HTML documentation, you will need to install the markdown package for python. Then, in the doc directory, run

    python make_doc.py > api_documentation.html

to generate the documentation.