Refactoring of restart strategy.
Restructure unit propagation. Parameter Solver stops at Variable::unitPropagation. Still working on.
Sort testcases.
Fix bug on deletion.
Remove cyclic dependency between solver, literal, variable, and clause.
Also declare the constructor Literal(Variable*) as explict.
Remove solver from literal and variable.
Move all the propagation in Solver. Literal has no more to do with propagation.
Merge branch 'master' of https://github.com/alviano/wasp