Styling controls in <spark-dialog>

@cleercode: look
@devoncarew: code

To begin with:

* Style the input text control in dialogs.
* Use Polymer's SVG icons in the Settings dialog.

![screen shot 2014-05-30 at 6 11 18 am](https://cloud.githubusercontent.com/assets/5606182/3130717/3b556f54-e7fc-11e3-90cd-e9d791436f9a.png)

![screen shot 2014-05-30 at 6 12 37 am](https://cloud.githubusercontent.com/assets/5606182/3130721/489e276e-e7fc-11e3-8070-80bda24c7425.png)