Drop ActiveSupport as a dependency

[paulgoetze]

Since only a fraction of ActiveSupport’s functionality is used (concerns, and a couple of String methods, like #blank, #demodulize, #(de)constantize) there is no need to rely on the full dependency.

Now uses plain

def self.included(base)
  base.extend(ClassMethods)
end

and

def self.included(base)
  base.class_eval do
    # ...
  end
end

instead of ActiveSupport::Concerns and implement the string methods itself.