fix for #152: allow users to style dialog types individually (second try) 

(this is a fixed version of my earlier pull-request (#153))

Added a class `alertify-<type>` to dialog. This way, the user can style one type of dialog different from the others: 

    .alertify-dialog {
      background-color: red;
      padding: 25px; 
    }
    .alertify-dialog-confirm,
    .alertify-dialog-prompt {
      background-color: blue;
    }