Implement 'to' 'until' and 'by' for RichDateTime etc

[azzoti]

If GenericRange becomes non experimental in Scala 2.8, then it should be possible to implement:
for (d <- DateTime.now to DateTime.now + 1.month by 1 week)

[azzoti]

GenericRange is now called NumericRange

[azzoti]

This request may not be possible using NumericRange having looked at what BigDecimal has to do to get to/until/by to work.