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
I think the options are
1)  SubjectBodyExtractor and FeatureUnion stay more or less as is
2) I replace them with ExtractSubject and ExtractBody classes and each element in the FeatureUnion transformer_list makes its own pass over the source data.

My bias is, of course, for 1.