Multiply confidence level for subjective document

A 100% confident positive document with 60% confidence in being
subjective should be different than a 100% confident positive document
with 90% confidence in being subjective. This closes gh-37.

We also revert "Use minimum objective or subjective probability", commit
2c6a8c9075023cb8fa7e0c94be4e181b22077310. As we now multiply the
confidence values together as if they were probabilities, we no longer
need to use the minimum confidence of each individual classifier.