Styling controls in <spark-dialog>

@cleercode: the look
@devoncarew: just as with the previous one, I'll submit for a code review after the preceding CLs land and I rebase.

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/5606182/3130808/4b8d0912-e7fd-11e3-82c8-ffda9b698abf.png)

![screen shot 2014-05-30 at 6 12 37 am](https://cloud.githubusercontent.com/assets/5606182/3130810/52b2a2c4-e7fd-11e3-9064-593690b2b814.png)