Add notebook on Games.jl/normal_form_game.jl

[mmcky]

Notebook for QuantEcon/QuantEcon.jl#95.

• The function cournot in cell [18] is type-unstable and is very slow for the case of N=4. With the current implementation, the input integer N cannot be used for type declaration (if I understand correctly). I have a type-stable version which uses @generated, but it is kind of advanced and so I hesitate to put it in a demonstration notebook.

@oyamad Apologies - I merged the wrong PR. This restores the PR and adds the latest version of the game_theory_jl.ipynb file from commit 9886d2865dbc75231ab93174e5cc55636f4cd23.

[oyamad]

This now uses Games.jl.

[oyamad]

The type-instability still remains for the function cournot in cell [18]. If we leave it as is for the moment, we can merge this now.