Bootstrap 3 Theme

Hi, thanks for the work being done to improve Alertify.

I added [Alertify Bootstrap 3](https://github.com/dbtek/alertify-bootstrap-3) theme in regard to a [demand](https://github.com/dbtek/alertify-bootstrap-3/issues/2) by Brad. This theme adds Bootstrap 3 styles for Alertify.
I am not quite sure if the master branch is right for this pr. Please correct me if I'm wrong.

Here, how it's look.
[![screenshot](https://github.com/dbtek/alertify-bootstrap-3/raw/master/screenshot.jpg)](https://github.com/dbtek/alertify-bootstrap-3)