Deprecate/rename DefaultSimilarityFactory to ClassicSimilarityFactory and remove DefaultSimilarityFactory in trunk

As outlined in parent issue...

• clone DefaultSimilarityFactory -> ClassicSimilarityFactory
• prune DefaultSimilarityFactory down to a trivial subclass of ClassicSimilarityFactory
  • make it log a warning on init
• change default behavior of IndexSchema to use ClassicSimilarityFactory directly
• mark DefaultSimilarityFactory as deprecated in 5.x, remove from trunk/6.0

This should put us in a better position moving forward of having the facotry names directly map to the underlying implementation, leaving less ambiguity when an explicit factory is specified in the schema.xml (either as the main similarity, or as a per field similarity)