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 tooltipScope