Ability to include custom restriction variable value and add the error message to base

Until now, the validates_timeliness gem can only use the default restriction value which is calculated from the variable that is specified in the :on_or_before, :on_or_after, :before and :after option. Now we can specify our own restriction_value which can be interpolated in the error message. 

This PR also adds the option of adding the error message to the base.