Merge pull request #2 from abate/master

Merge Pietro's commits introducing ocamlbuild.
Reinstated Menhir as our parser generator. Updated the README file to fit the current dependencies (i.e. removed Batteries).
Cleaned up a little the Generic_constraints and Variables modules implementation. Updated the src/datatypes/data_constraints.ml* with new constraints code.
First try of adding intermediary internal types representing input.
Add Makefile