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)

- Implement smt and smt-z3/smt-cvc4 as options for --solvers

- Uses a main solver sources for verification/CLP, taking into account
  --solvers

- Implement Z3-SMT sets

- Add tests for Z3-SMT