Remove some redundant rdoc in Rails and Sinatra extensions
nodoc the sinatra integration capture method
Add Form#raw_output method, for handling automatic html escaping

This makes the Rails integration use this, which simplifies some
of the Rails code and should make integration with other web
frameworks with automatic html escaping easier.
Add Form#serialize_open and #serialize_close private methods
Do not extend objects at runtime when using the Sequel support

This makes Forme::Form.new delegate to a subclass if there is an
object given and that object implements forme_form_class.