Requirements.Normalize()
Improved Solvers architecture

--HG--
rename : src/Solvers/SimpleSolver.cs => src/Solvers/BacktrackingSolver.cs
rename : src/Test.Backend/Solver/SimpleSolverTest.cs => src/Test.Backend/Solvers/BacktrackingSolverTest.cs
rename : src/Test.Backend/Solver/ExternalSolverTest.cs => src/Test.Backend/Solvers/ExternalSolverTest.cs
rename : src/Test.Backend/Solver/SolverTest.cs => src/Test.Backend/Solvers/SolverTest.cs