Added support for popover-html directive for binding html

The [popover](http://getbootstrap.com/javascript/#popovers) component in Bootstrap supports using html as your content.  This proposed fix creates a new directive, `popoverHtml` to support this new behavior.  You can observe this directive at work [here](http://plnkr.co/edit/vl0oalW5bpJ1ss67c2rm?p=preview).

It modifies the tooltip script to conditionally choose the template used depending on whether an optional fourth flag is true.

There is a caveat of some code duplication due to reusing most of the HTML from the current popover directive.

It may not be the ideal implementation, and would like thoughts on this.  However, I believe that it would be ideal to try to offer this capability for popovers, simply since the current non-support is a regression.