also:
- fix noosfero/i18n: error_messages_for should works as documented in
  Rails API (accept param :header_message => '...')
- new helper method to display errors in a dialog box
- now Plugins knows what is URL to admin itself
- defining paths to cucumber steps and setup files in config/cucumber.yml
- allow plugins to have documentation in same way as Noosfero core

(ActionItem2056)