Add validates_absence_of matcher

Adds a validates_absence_of matcher. The tests are written assuming that pre-Rails 4 projects will be using validates_inclusion_of or custom validations instead of non-existent absence validations. The `validate_absence_of` matcher will work for them as long as they use the `:present` message for their validations or use the `with_message` method to change what message the matcher looks for.