Add automated tests

Interpreter class now has a handy test() function which is called by
Interpreter::main(). This method finds all the files ending with .cpp in
tests/ directory and run test() on each of them.

	Any error from the interpreter is caught and reported
	on stderr (on terminal).

	Interpretation result will be written to
	"tests/src.interpretation.txt"
	where src is a cpp source file under tests/.

Just run the follwoing command after compilation

	`$ java Interpreter`