Add presence query methods for associations.

Allows associations be queried for presence just like attributes:

    class User < ActiveRecord::Base
      has_many :tools
    end

    user = User.first
    user.name? # => true, regular attribute query method
    user.tools? # => true, checks whether user has any tools

These are equivalent:

    user.tools?
    user.tools.present?

Motivation:
When extracting model attributes into models I have to change all `foo?` calls into `foo.present?`.
With this change it's more uniform and less painful.