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)

Added more documentation

(b) Adding support for inferring time, depth, stack and rec bounds
(c) Adding support for compositional reasoning in inferrence of time bounds
(d) Adding support for estimating stack usage at runtime
(e) Adding support for fractional literals

The test target `isabelle:test` now executes cleanly on Windows,
including bootstrapping.

Split SMTLIBSolver to Solver+Target to allow things other than leon
solvers to talk SMT.

unit, integration, and regression tests can now share code as they are
compiled together.

Unit tests are now in package leon.unit and shared code lives in
leon.test.