[Proposal] Implement JsonSerializable Interface

[Domi04151309]

It would be nice if the result could be converted to JSON by having the classes implement JsonSerializable.

[uestla]

The result can be simply just by json_encode($solver->getSolution())

[Domi04151309]

Yes, that's right. What I meant was being able to call json_encode($solver) to get a JSON-Object containing all public variables (accessible through the getters) in a similar format to print_r.