add bootstrap-popover as display option for help

Like in #401 I'd like to use a popover
(http://twitter.github.io/bootstrap/javascript.html#popovers) for my
help text, as I need some well formatted title/content inlcuding some
HTML-formatting.
Merge pull request #422 from Mercoline/helpLabelPopover

add bootstrap-popover as display option for help