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)

Performance tests are invoked via "sbt perf:test". Currently testing
performance of CEGIS on some random synthesis problems

--solvers=smt-cvc4-cex
--solvers=smt-cvc4-proof

Update scala-smtlib with support for parsing special DefFunsRec

Fixes a bug, where UnrollingSolver was interrupted but was not
stopping its internal solver, leading to a timeout not being
respected if the internal solver was stucked in a complicated
check operation. UnrollingSolver now takes a Solver with
Interruptible.

In order to do that, Leon now relies on a more recent version of
scala-smtlib, that provides a feature to kill the solver process
in a relatively clean way. Update the SMTLIBSolver code to
actually perform the kill operation on the interrupt.

reactivate tests for CVC4, if available. Remove non-linear testcase that
worked only on z3 (SumAndMax)

Update the build to reference the most recent version of
scala smt-lib and adapt the code to the new API.

Replace some use of NonStandardCommand by built-in standard
Commands of scala smt-lib.

 - Partial support for CVC4

- 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

- Implement the Leon Library in Leon-land rather than Scala-land.
  import leon.Utils._ becomes import leon.lang._
  import leon.Annotations._ becomes import leon.annontation._

  For now, the library defines generic Options and Lists.
  The library is automatically imported from the ./leon script, unless
  the --library=no option is passed.

- Support parsing of multiple files and modules.

- Introduce new annontations:
    @ignore: remove definition from Leon
    @verified: do not consider for verification unless explicitly
        specified

- Free&Recreate solvers during synthesis. This avoids huge memory leaks
  due to Z3AST never being reclaimed thourough the entire synthesis
  process

- Add safeguard to catch Z3Solvers for which memory management is
  incomplete

- Refactor code extraction
  - Standardize extractors
  - Fix type identification by resolving type aliases

- Redo reporter integration with Scalac
  - Fix relining to prevent bizarre compile-errors

- Refactor Pretty-Printers
  - Extensible class interface
  - Not returning mutable string-buffers anymore

- Fixes according to 2.10
    - 100% false warnings in Tests
    - imports for postfix-ops or implicits

- Upgrade ScalaZ3 to 2.10

- Upgrade Cafebabe to 2.10

Improve performance of FairZ3 by lowering unrolling to the z3 level.  Improve synthesis profiling script.

Improve performance of FairZ3:

  Make function templates and unlocking/unrolling work directly at the z3 level for performance reasons.

  Implement push-pop at the unrolling-bank level. Works around a z3 bug.

  Z3 apparently side-effects during check-assumptions, causing a following
  check without assumptions to produce unreliable results. We work around this
  by backtracking to the state before the check-assumptions using
  push/pop.

Improve synthesis profiling/benchmarking:

  Move benchrmarking suite away from tests. Synthesis Rule application
  benchmark can now use a --rule option specifying what rule to apply.

  Add Testcase to evaluate CEGIS unrolling of different depth.
  Benchmarking script is now generated via the 'bench' sbt task.