Multiple updates

- Experience moved in MDP namespace.
- Added default values for discount parameters in MDP models.
- Added discount parameter in basic constructor of MDP::Model.
- POMDP constructor now takes its parameters plus the ones taken
  from its parent class. This allows finally for freely inheriting
  POMDP::Model from any correct MDP model class.
- Added the IncrementalPruningTests.cpp file. As of now it is not doing
anything, but I am using it to debug a variety of classes to finish up
the IncrementalPruning algorithm.