comment a lot and cleanup the interface a bit
add a .odocl file for documentation generation: ocamlbuild generator.docdir/index.html
relicense under BSD

Simon Cruanes expressed his interest in incorporating bits of
random-generator in his QCheck library
( https://github.com/c-cube/qcheck/ ), but he's too afraid of
hereditary licenses to properly reuse GPL work. I'm relicensing this
under BSD (after checking with Xavier) to make everyone's life easier.