added simple gender_to_human method #40

This is implemented as additional module which must be opted-in,
either on a per model basis

    require 'simple_enum/contrib/humanize'

    class MyModel < AR::Base
      extend Simple::Contrib::Humanize
      as_enum :gender, ...
    end

    @my_model.gender_to_human # => "Translated!"

or globally, based on the ORM, in initializer:

    require 'simple_enum'
    require 'simple_enum/contrib/humanize'

    ActiveRecord::Base.module_eval do
      extend SimpleEnum::Contrib::Humanize
    end