Changed methods .pop and .rotateLeft, .rotateRight

Changed .pop to remove the reference to the object. Changed the default value of .overflow to null. Changed how the default value of rotateLeft/Right is detected.

The .pop method should remove the reference from the buffer, because otherwise if it's a buffer of objects they'll never be garbage collected. Since overflow is a function object, it makes sense that it starts out as null. The methods rotateLeft and rotateRight should support receiving 0 (!cntr would evaluate to true and change it to 1 otherwise).