change the preview button

Change the markdown preview button to be a round polymer button like the outline button; give it similar margins in the editor (again, like the outline button). Fixes https://github.com/dart-lang/chromedeveditor/issues/2711.

@financeCoding

![screen shot 2014-07-04 at 9 56 12 am](https://cloud.githubusercontent.com/assets/1269969/3494198/bfb5d2ec-05c1-11e4-91a3-437dbfa02606.png)