Support for Object.from_param

[andruby]

A small patch to allow models to define a from_param method that can be used for slug/permalink urls.
Define both to_param and a self.from_param(id) methods in your model and get nice url's for free!

example model:

class Person
  include Mongoid::Document
  field :name
  validates_uniqueness_of :name

  def to_param
    name
  end

  def self.from_param(param)
    self.where(name: param).first
  end
end

[mrmemes-eth]

Thanks for the pull request. This seems fairly application specific though, so I won't pull it into master. As of right now this is probably something I would personally use, but I would opt for overriding the default_exposure instead.

Thanks again!