Add function to get the weight of a container to lua script

This should add a Container:getWeight() to lua script.
I would rather just get rid of the use of floating point numbers everywhere and use integers, we don't really care about precision beyond two decimals anyway and add/sub operations on ints are faster than on floating points. When representing it as a string we can divide it by 100, or perhaps even better, just convert the int to string and insert a dot before the last two digits. This would also provide better correctness, we wouldn't have to worry about any rounding problems in floating points where the number we're adding up to can't be precisely represented as a floating point number.