Fix the initialization of lastState

There is however an error remaining (boost::optional data is
destroyed at some point, resulting in the impossibility to get
the last state of a SolverError. Investigation is in progress).
Add a test for the result value of a SolverError
Update documentation for DummySolverLastState.
Update CMakeLists.txt meta-information.
Merge pull request #4 from bchretien/master

Fix master branch and add a unit test with lastState