Improve build system and add the CompCert parser.

This commit includes the following changes:
* New scons builder for Ocaml. It supports ocamlc, ocamlyac and ocamllex.
  Support for native code compilation via ocamlopt remains to be done.
* Use the Ocaml builder to compile coq2html
* Extraction now generates ML modules in the extraction directory, scons
  uses the Ocaml builder to compile these.
* Add the (old) CompCert parser, we will later attach it to the frontend
* Update LICENCE
Update README.