Merge pull request #172 from MikaelMayer/on-the-fly-string-conversion

src/main/scala/leon/purescala/Common.scala

src/main/scala/leon/purescala/DefOps.scala

src/main/scala/leon/purescala/Definitions.scala

src/main/scala/leon/purescala/Expressions.scala

...cala/leon/solvers/combinators/Z3StringCapableSolver.scala

src/main/scala/leon/solvers/z3/Z3StringConversion.scala

src/main/scala/leon/utils/Bijection.scala

src/test/scala/leon/integration/solvers/SolversSuite.scala