Fix BUG:Getting strategic contingency for game in extensive form

Fix issue #60.

The crash reason of getting outcome in extensive-form game is that in
C++ part(TreePureStrategyProfileRep) doesn't implement the GetOutcome
function(just throw UndefinedException).

The current solution now is creating a Cython-level TreeGameOutcome
which wrapping the C++ TreePureStrategyProfileRep class stands in for
an outcome in the extensive-form game.

I have resolved it according to your giving solution in the issue page.
Please review it. Thanks.