Fix button styles in Firefox for Android

Button elements in Firefox for Android need `background: none` to override background gradients. In the docs, the menu button and `material-sidenav` buttons did not have this style--this PR has a fix. See attached screenshot of the issue:

![Firefox Android buttons](https://cloud.githubusercontent.com/assets/2712395/4537621/80b2c246-4de0-11e4-8e3a-ef9444d1ecbb.png)