Logging system improvements:
- Changed PrintToFile_Threaded and PrintToFile_ThreadManager to RunHandler_Threaded and RunHandler_ThreadManager to take advantage of the thread logic for any handler (including user-provided handlers)
- Added backtrace functionality and tested on Windows and Linux. Not tested yet on os x so I'm not checking this in yet.
- Added extra macros: LOG_IF, LOG_EVERY_N, LOG_IF_EVERY_N, LOG_FIRST_N, LOG_ONCE, and LOG_ASSERT
-Added thread id in log messages, slightly changed default format