SOLR-1365: New SweetSpotSimilarityFactory allows customizable TF/IDF based Similarity when you know the optimal "Sweet Spot" of values for the field length and TF scoring factors (merge r1450577)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1450579 13f79535-47bb-0310-9956-ffa450edef68