New java Api Union

Corr. Issue: https://github.com/stratosphere/stratosphere/issues/621

A union operator for the new Java-API callable: dataSet1.union(dataSet2)
There exists a binary union operator from the java-api down to the optimizer level. 
This means that unions do not need to be handled over lists of operators by the commons-api anymore, but a cascade of union-operators can be used for this case. Therefore my next step is to clean the code from the operator lists to make the code "slimmer".