New version of Frama-c, Ocamlgraph, Why2. Warning: renaming of Frama-c version

Frama-c version where not correctly compared as discussed in #594. I made the choice to rename them for opam:

* framac.Oxygen-20120901 became framac.20120901
* framac.Nitrogen-20111001 became framac.20111001
* ...

It can be strange for the user during update but it seems the only simple way.