Look up and use best + worst bets at search time

Add metasearch index to app.  Pass it through UnifiedSearcher to
UnifiedSearchBuilder.  Use a BestBetsChecker to look up bets from
UnifiedSearchBuilder, and build a query which obeys the bets if there
are any.
Add disable_best_bets debug option

This adds a generic debug parameter, which accepts a comma separated
list of options. There are plenty of options we can add in future, such
as providing access to elasticsearches explain API so we can understand
the weighting better.  For now though, just add an option
"disable_best_bets" which disables the use of best bets for query
ranking.
Use a custom analyzer for the stemmed best bets

We want an analyzer that does stemming, but not stop wording or
synonyms.