Fixed some object having different behaviour than the pure-data ones and fixed some bugs

Add BANG message support for objects MessageAdd, MessageSubtract, MessageMultiply, MessageDivide and MessagePow so they have the same behaviour that the pure-data ones

Fixed MessagePow returning wrong values for a power <= to 0

Fixed some other bugs aswell
I would suggest instead the names `constant` for the "right operand", and `last` for the "left operand".