Change default scores in the ranker to the largest negative number.

This is necessary e.g. for the language model rankers, where all scores
will end up negative (sum of log probabilities, which are always negative).
Use std::numeric_limits<double>::lowest() instead of min().

std::numeric_limits<double>::min() doesn't do what you would expect.
Set initial score to 0 upon finding a document matching a query term.

Fixes bug introduced in previous commit.
Merge branch 'develop' into knn
use std::numeric_limits<double>::lowest() instead of min()