FeatureUnion example using a heterogeneous datasource.

Uses the 20newsgroups dataset. Extracts & processes the subject-line and body
with separate pipelines and combines them with a FeatureUnion.

Closes #3569
Maybe to not confuse this with element lookup at all, we should be getting things out of dataframes/recarrays using getattr instead... I don't really know what the right solution is to make everyone happy.