~ Improve csrf protection

Add token to a_post helper;
replace base64 to urlsafe_base64 in csrf_token
~b Escape url segments
+ confirm option for a, a_post

  #{a 'link caption', :action, :confirm=>'Are You sure?'}
  #{a_post 'link caption', :action, :confirm=>'Are You sure?'}
+ built-in link to Sequel hooks documentation
~ embed config into controllers, wrappers and helpers
. Comment fix