-PrincessSolver.scala
 contains the wrapper code of an (incomplete) api towards princess.
-princess.jar & scala-actors-2.11.0.jar
 A (special) version of Princess with support for Natural numbers
 (i.e., an ADT with Succ, isZero, Pred and constant zero.

Also changed SolverFactory s.t. it allows --solvers=princess to run
with the new Princess backend.