Add SORT constraint

- add PropSort (+ correctness and consistency tests, ICF, etc.)
- Add Solution#toString()
Merge branch 'feature-sort' into develop

Conflicts:
	choco-solver/src/main/java/solver/constraints/IntConstraintFactory.java
	choco-solver/src/test/java/choco/checker/Modeler.java
	choco-solver/src/test/java/choco/checker/correctness/TestCorrectness.java