initial commit minisat2 branch

- add ocamlbuild-based build system
- decouple the library from minisat source code
- add a dependency to minisat shared library (libminisat)
- add ocamlfind META information
object interface and cleanup

- libminisat_stubs.c :
  + remove global variable solver. Now we can create as many instances
    of solver as we want.
  + general cleanup

- minsat.ml :
  + change value from int to variant
  + add object interface

- minisat.mli :
  + documentation

- solver.ml :
  + remove type annotations
  + make it compile with the new object interface
fix minor compilation problems

- add compile & install notice in README
- fix header to make it compile cleanly on debian sid (need the minisat2
  package installed on the system)
- add binding for solver::conflict
- make it compile with with minisat2 (stage commit)
- stage commit
- fix various problems. now it seems working
- add note in the README
- fro lit list to lit array.
added list_sign and lit_to_var & cleanup
Changed includes to reflect the minisat include hierarchy, as set up in the resolution of the Debian bug #606643.
Added a missing parameter to the declaration of minisat_mklit.
Uncommented linking with libstdc++.
Updated Val_clause to match the most recent minisat API.