add enumerate_states.hh for callbacks on each new lm state that is read during lm loading

example callback class for states:

/**
 * Receives callbacks from KenLM to build states
 */
class ActualStatesKenLm: public lm::EnumerateStates {
public:
    ActualStatesKenLm(std::vector<std::vector<word_id_t> > * states);
    virtual ~ActualStatesKenLm();
    virtual void Add(std::vector<word_id_t> state);
private:
    std::vector<std::vector<word_id_t> > * states;
};

--

- configure kenlm with callback
    config.enumerate_states = new ActualStatesKenlm();

- load lm
    ...