Custom FilterTermExtractors should be added to the beginning of the FQE extractor list

They were being added at the end, and never getting called because the Generic
filter extractor got there first.

Fixed #25
Move Filter tree term extraction to new FilterTermExtractor class

Most of the logic of FilterQueryExtractor moves here, and is then shared with a
new ConstantScoreQueryExtractor.
Ensure that empty term lists are weighted zero