Wm notes 1.15.0

Pinging @mglukhovsky (and @larkost if he wants to check my checking on pull requests and issues).
While "up to 50x" is true, we should give more detailed numbers. Based on https://github.com/rethinkdb/rethinkdb/issues/1915#issuecomment-53115261 I think we should say ~1.15x faster for simple queries, ~2x faster for analytical queries, and ~50x faster for count queries.