Extensions homogenization

This PR is in relation with ocsigen/ocsigenserver#43 - Eliom is also an extension. Perhaps there is a problem with the documentation (I don't know all the intricacies of Eliom), if you could see.

There is a factorization between the function of parsing the configuration file and the `init` function. Of course, the request scheduling of Eliom on Ocsigenserver is not yet programmed, it will be another PR for statically linking between Eliom and Ocsigenserver (and MirageOS, oups).
No space here, and this should go in a .mli