adding support for printing stack traces using terralib.traceback() e.g.:

terra foo()
	terralib.traceback()
	return 1 --this would tail call eliminate if not here
end

it is run by default when the program segfaults but right now
it is missing the crashing frame which has to be recovered using a process similar to:
http://stackoverflow.com/questions/77005/how-to-generate-a-stacktrace-when-my-gcc-c-app-crashes
support for printing segfault tracebacks. still needs debugging on linux.
updating luatjit version to 2.0.2
replacing backtrace with a custom function that might work better on linux.
changing interface to traceback to take optional pointer to ucontext_t