Many improvements in tactics (and tactical) representation:

Many improvements in tactics (and tactical) representation:
1) tactics are no more functions from state to state, but functions from
   proof * goal to proof * goal list where the goal list is the list of
   new goals generated
2) proof and goal are no more optional
3) all the tactics have been slightly changed so that their type is now
   param1 -> ... -> paramn -> ProofEngineTypes.tactic
4) the tactical thens has been implemented

Other changes:
1) more .mli committed
2) comments and copyright notices added where missing