Remove ActiveModel and ActiveSupport

The benefit from depending on ActiveModel and ActiveSupport are too small too keep them. They are very big dependencies. `validates_presence_of :frequency` isn't particularly useful, because

1. it will always be set, according to the constructor,
2. users won't call `#valid?`, because it just doesn't make sense in this project

All of this `#blank?` and `#present?` that I've replaced with `#empty?` are valid, because these variables will never be `nil`, since blank regular expression matches always return empty strings instead of nils.