fixes #152
    - added nice validation to all the form (including the email field) using the ckan's form macros mechanism
    - added captcha to better protect form agains multiple submission attacks - the captcha api code needs to be added in the development.ini
    - new link under "Sysadmin settings" (need to be logged in as admin) so that the admin can add new users to CKAN
fixes #152
    - nicer way to extend the layout :)