Changeset 8678 for code/branches/unity_build/src/libraries/util

Replaced TriBool with a modified version of boost::tribool. The main reason was that 'True' and 'False' had to be replaced because of macro collisions.
So why not use the real true and false with a third state? That's what boost tribool did, so I changed that implementation to fit our needs and change its behavior.
It's also safer to use because it avoids unwanted implicit conversions.