feat(popover): support templates (WIP)

This is a proof of concept to use a tooltipController to transclude a template.

It's based off #1600 and most of the changes are from that. See https://github.com/chrisirhc/angular-ui-bootstrap/commit/0a1510c957bd385db61dcb1a8366fef97fd38e74 for actual changes for the popoverWindow.
![record](https://f.cloud.github.com/assets/132584/2243498/a3e7f958-9d2c-11e3-9a2f-ae1bea980598.gif)



Tests are missing but I wanted to open this up for discussion or comment.

Known issues:
- Doesn't support changing of the template URL (there's no watcher)
- Doesn't support live updating of the positioning after the template has been reloaded

These known issues can be fixed through adding methods or watchers to the tooltip controller.
Should be using clone function