back to 1thread writer; del customSimilarity

* in general it seems that if multiple threads are needed to index the
  data, the RAMDirectory() scenario is already a failure. so there is no
  need to add complexity for nothing.

* seems that having arguments to the Similarity is not as usable as i
  thought, so just removed that, and moved all the parametarized things
  into CustomQuery interface with parameters

* add CustomScoreQuery:
  custom_score => {
    query => {...},
    params => { a => { b => c }},
    class => "MyCustomScoreQuery.."
  }

* some fixes:
+ groupEndField was wrong
+ increase the default searcher size to 100 (up from 10)
+ add indexlevel option to chose default index similarity
+ add total_grouped_count, total_hit_count and total_group_count in the
  resultset
+ fix boolean query to start with minimum_should_match = 0