Scale microphone image margins.

When the mic button is scaled, the margins are not, causing the microphone image to be out of proportion with the button (see image). This pull request fixes the issue.

![ios-simulator-screen-shot-16-apr-2014-20 33 26](https://cloud.githubusercontent.com/assets/645674/2726434/d61c4878-c5ba-11e3-9097-e32b2b983426.png)