default accumulators - sum, product, joinOn

ragarding "sum" and "product": do you think this is a good implementation?

pro:
- totally generic accumulator
- no params required (easy to use)

con:
- not all T's are supported (i.e. only the final subclasses of java.lang.Number in java.lang.*)
- involves lots of conversion and casting (looks clumsy, should be typesafe though)