AccentedSquareButton, IdealForegroundColor key and hover-effect for Dialog buttons.

- Added an 'AccentedSquareButton' style 
- Updated every accent to have an 'IdealForegroundColor' key. 
- Updated Dialogs to use that so that the accented button would have a hover-over effect.

### Screenshots
![b1](https://f.cloud.github.com/assets/251501/1673873/5c96190e-5cef-11e3-94b3-3c60bdd3ea13.PNG)
![b2](https://f.cloud.github.com/assets/251501/1673874/5c9a1bc6-5cef-11e3-8615-8f3ea3a43c3d.PNG)
![b3](https://f.cloud.github.com/assets/251501/1673875/5c9e8170-5cef-11e3-9dc4-a34c4da8da16.PNG)