iOS 6 Compatibility

This is probably just a temporary adjustment since most apps might drop support for iOS 6 by the end of this year. So I merely altered some iOS-7-only methods instead of making changes to the UIToolbar layers mentioned in #29.

Unfortunately, UILabel [`adjustsFontSizeToFitWidth`](https://developer.apple.com/library/ios/documentation/uikit/reference/UILabel_Class/Reference/UILabel.html) doesn't work well in iOS 6. 

> In iOS 6 and earlier, this property is effective only when the numberOfLines property is set to 1.

But I think that won't cause too much trouble.

![img_3820](https://cloud.githubusercontent.com/assets/2032500/2689107/433abb06-c2e6-11e3-97e7-730e5c63c612.png)