Fix print for Solver and SolverState.
Add unit test for SolverState.
Set SolverState as noncopyable.

Fix roboptim/roboptim-core#43
Add an example based on the one if the documentation.
Fix constraint print in Solver.
Fix indent in problem.hxx.
Update example in documentation.
Fix signed/unsigned comparison error.
Merge pull request #45 from bchretien/master

Improve SolverState (test it, make it non-copyable as suggested by #43), close #43.