Isolate popover directive scope

Previously, if multiple popovers were added, they would all share the
same scope.

I encountered an issue when trying to add several popovers, each with a
"close" link that specifying `ng-click="hidePopover()"`. Since all
popovers share scope, the `hidePopover()` function only knows about the
last popover it saw. This means that only that last popover can be
hidden using `hidePopover()`.

This change ensures that all popovers have their own scope, so
`hidePopover()` will now operate on the correct instance.

Fixes #11.

Demos (try clicking the “Close” link inside popovers):
* Before patch: http://plnkr.co/edit/R53RPA7EdRkiU9NvB1D9?p=preview
* After patch: http://plnkr.co/edit/QpPzcBDm0OBMdUejFGpv?p=preview