0.6

CHANGES:

• migrate from minisat-c 1.14 to minisat 2.2 (in C++); refactor it to build with C++11

• add Lit.{apply_sign,hash,equal,compare}

• add ensure_lit_exists

• do not call simplify implicitly before solve

• add value_at_level_0

• add unsat_core

• add okay