Add Pi approximation Java example

Hadoop and Spark have a Pi approximation basic example (similarly to WordCount, PageRank ...), so for homogeneity it is worth having it in Flink as well.

Note:
The final result, count, which in this case is the approximation of Pi had to be assigned the type DataSet<Double> instead of just Double (due to Flink limitation)
So aside from executing a summation, the reduce (at the collector level) also does a little math operation, which could have not been applied on type DataSet.