Generalised the name of the expected InputSplit class in HadoopDataSource.
Add avro to stratosphere-dist, thus adding it to the lib/ folder
Better estimations for filter operator
Corrected minor errors in the optimizer filter operator.
Add getSolutionSet() and .getWorkset() for delta iterations

Before, the result of dataset.iterateDelta() would be the workset and
the solution set could be obtained via iteration.getSolutionSet().

This is confusing since you are not told what the result of
.iterateDelta() is supposed to be. Now, when using the result an
exception is thrown during pre-flight that advises the user to use
the two methods to get the correct dataset (work, solution).
Rename DeltaIterativeDataSet -> DeltaIteration, is no DataSet anymore

Now a user cannot mistakenly use it as a data set but is forced to use
the .getSolutionSet() and .getWorkset() methods.