-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.

anyway)

Corrected bug with unicode chars in the build.sbt

Corrected bug with unicode chars in the build.sbt

Notable changes:
* sbt-libisabelle plugin which takes care of Isabelle source management
  - no more submodules; Isabelle sources are now packaged in JAR files
  - no weird ROOTS file in the repository root
* less isabelle: flags, everybody would want to use the defaults anyway
* updating to Isabelle2016 becomes possible (future work)