Allow #count to take $hint option?

[ryana]

Should this line also take into account options so that hints like this can be used:

=> #<Mongoid::Criteria
  selector: {"organization_id"=>BSON::ObjectId('556f2a9b77656271580c0000'), "last_active_at"=>{"$gte"=>1970-01-01 00:00:00 UTC}, "created_at"=>{"$gte"=>2015-08-24 23:56:58 UTC}}
  options:  {:hint=>{"organization_id"=>1, "type"=>1, "last_active_at"=>1, "created_at"=>1}}
  class:    CommunityMember
  embedded: false>

I'm more than happy to supply a PR w/ tests if so. Thx.