Change popover/tooltip-template to evaluate expression

[chrisirhc]

Must do this before 0.13.0 release, as it would be a breaking change if we do this after release.
Before:

popover-template="template.html"

After

popover-template="'template.html'"

The reason for this is to support $sce trusted urls. In order to pass a trusted URL, one needs to use an expression.

In JS:
$scope.templateUrl = $sce.trustAsResourceUrl('http://good.com/template.html');

In HTML:
popover="templateUrl"