Use std::unique_ptr on gnmgame class

Please change line in main Makefile:
CXXFLAGS = .... -std=c++0x

This is my first Gambit pull request and an initial update to C++11 STL. Only one file is class is modified to use unique_ptr. The other files mentioned as candidates in ticket #112 are more problematic. I believe there is an issue of inheritance, so lots of function definitions will need to be changed.