Option to display only one popover at a time

See issues:
https://github.com/angular-ui/bootstrap/issues/618
and
https://github.com/angular-ui/bootstrap/issues/590

This feature allows for a single popover being displayed. Any new popover will close any older popovers. Clicking on body will also close the popover. The behaviour is optional and by default the popover will retain its existing multiple display functionality.

This feature can be turned on via:
$tooltipProvider.options({ popoverMode: 'single'});