Player creation interface enhanced
NormalFormGame generalized to the N-player case
Some more tests added
payoff_vector method added.
Allows data input with ndim=2 for a symmetric two-player game
Now do not allow a Player to be created with action sizes

This is because if the input data has ndim=1, cannot determine whether it is `action_sizes` or it is a degenerate game with one player.
Consistency in the input array checked
Minor edits
Tests for invalid inputs added
payoff_vector refactored; br_correspondence deleted
Merge pull request #7 from oyamad/N_players_rev

New interface for Player and NormalFormGame creation
Degenerate cases with one player handled
Minor modification
is_nash: two-player case handled separately
is_nash: use ndarray in case with more than two players
NormalFormGame: consistency of number of players checked
NormalFormGame: construction via data with ndim=0 corrected
Bug in is_nash corrected
Minor modification reverted
test_getitem added