Add 'interior-point' option to is_dominated; add dominated_actions

[oyamad]

import quantecon.game_theory as gt

player = gt.Player(
    [[-1, 1],
     [1, -1],
     [-0.5, -0.5]]
)
player.dominated_actions()

[2]

player.is_dominated(2, method='interior-point')

True

[coveralls]

Coverage increased (+0.02%) to 95.179% when pulling 89e1b4a on is_dominated into 8657c91 on master.