Exp4.P cannot handle delay reward

[ianlini]

Maybe we should use the score in history, but not store it in model.

[yangarbiter]

This algorithm is originally not designed for delayed reward.

[ianlini]

linucb is also not, but we modify it to make it do it...

[ianlini]

The current implementation of exp3 also doesn't support delay reward

[ianlini]

#108 is the solution for exp3