Dialog Provider

This PR replaces the [previous one](https://github.com/angular-ui/bootstrap/pull/25) I closed.

I moved all dialog work to a [feature branch](https://github.com/typesafe/bootstrap/commits/dialog), and then rebased and squashed it all into a [single commit](https://github.com/typesafe/bootstrap/commits/master) on master.

I also updated the code to use the new `$transition` service.