Added Log class

- Log class will output to a file (Game.log currently) if the current
log level passes the filter level.
- Added _DEBUG, _WIN32, and _LINUX preprocessor defines.
- Changed VS2010 Debug builds to SubSystem=Windows to remove the console
window; the log file will automatically print output to console window
(Linux), or VS debug output pane (Windows) ONLY when built with _DEBUG
defined.