Add data-hide attribute to the bs-popover directive

This allows callers to specify a scope variable that, when truthy,
causes the popover to be hidden. Very useful when you need to dismiss
a popover from its parent scope.

Usage example:

```
    <button bs-popover="'/my/partial.html'" data-hide="foo">Click Me</button>
```
(when $scope.foo becomes truthy, this popover will be hidden)