Add count method to DataSet and implement CountOperator

At the request of @twalthr. This is the count operator I've implemented some time ago to get the to know the new Java API. It introduces `DataSet.count()`, which is executed as a map (to ones) and reduce (sum up the ones). I initially didn't do the PR, because of the following problem: empty DataSets don't work as the first map won't have any input to operate on.

If more people think that we should include this operator we can think about a possible solution to the problem.