First step for implementation of Issue #69

I have added the suggested functionality. I tried to stick to the existing coding style as much as possible.

I have added the utility method `FieldUtils.divideBy`, even though this is used in only one place at the moment. If this is not wanted then it can be inlined easily enough (some tests have to be moved though). I have not provided `int` overloads here, as they are not needed at the moment.

Normally I would have documented the possible `ArithmeticException` on the APIs, but this was not done for any of the other methods, so I stuck to the existing convention. Maybe this should be reconsidered?

When reviewing please check that I have not overlooked any corner-cases in the division.