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
This is not clearly the purpose of `DictVectorizer`. To use that to select a single column, you would need to hack it to have a vocabulary of size 1 and it would still return a 2d array that I'm not sure one can pass to `TfidfVectorizer`.