Added overload to support custom frame.

Custom frame enables easy use to display the scanner in a UIPopoverController instead of showing it full-screen. An example can be found in the [zxing-ipad-example](https://github.com/krummler/zxing-ipad-example) project.

![screenshot](https://f.cloud.github.com/assets/1032307/145269/7a2aead6-745a-11e2-8fe3-7ee71b1b9306.png)