Overlay Module

This adds a generic Overlay modal dialog that can be used for lightboxing other components.

Screenshots:

![overlay-loading](https://f.cloud.github.com/assets/518943/749287/df5a4688-e4a6-11e2-8026-ffc338792df1.png)
![overlay-demo](https://f.cloud.github.com/assets/518943/749288/df6b97bc-e4a6-11e2-9857-794d13132889.png)