New-Java-API Union

First PR: https://github.com/stratosphere/stratosphere/pull/674
Corr. Issue: https://github.com/stratosphere/stratosphere/issues/621

Now the old-java-api is also using the binary-union-operator by automatically creating a cascade of unions if needed. Therefore every operator in the common-package saves a single operator instead of a list of operators.

But the scala-api has some parts where it expects list of operators for getInputs() calls. Therefore there are getInputs() methods which are wrapping lists around the single operator and return the list. This is a little be ugly, but I assume we switch the scala-api to use the new java-api in the future, so we can remove them then.