Method as keyword (not only fields)

Rewrote  Mongoid::Search::Util.keywords, now it can be used with methods as well.
Also, it has to work with more nesting levels than one (not only with parents, but also with grandparents), but  it didn't because of mongoid's callback order (durran said he will rewrite it in 4.0.0).
I've added some tests to describe this situation.