Add support for diagonal/corner positioning of popovers

top-left, top-right, bottom-left, and bottom-right.

Unfortunately, these corner positions are not the combination of their components.

![screen shot 2014-11-21 at 12 24 21 pm](https://cloud.githubusercontent.com/assets/174740/5149043/95827966-7179-11e4-84fc-33c1bb9f6db1.png)