Improve of SVG buttons in hover/no-hover

@gaurave 

This especially improves the closing X in `<spark-dialog>`. Below are non-hover vs. hover comparisons:

![screen shot 2014-06-05 at 12 10 29 pm](https://cloud.githubusercontent.com/assets/5606182/3192347/2b1f9ed2-ece5-11e3-8120-5712a9446b60.png)

![screen shot 2014-06-05 at 12 10 36 pm](https://cloud.githubusercontent.com/assets/5606182/3192352/3442a40a-ece5-11e3-87bd-0e7ad15e5dfc.png)

![screen shot 2014-06-05 at 12 10 51 pm](https://cloud.githubusercontent.com/assets/5606182/3192354/392fa954-ece5-11e3-892a-560c13d3f5bb.png)

![screen shot 2014-06-05 at 12 11 00 pm](https://cloud.githubusercontent.com/assets/5606182/3192356/3d86c546-ece5-11e3-81e1-1b11d3bd3a31.png)