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.
Merge pull request #12 from teddywing/isolate-popover-scope

Isolate popover directive scope