Fixed AVCaptureMetaDataOutput example in iOS 7 article

It is only hinted at in the documentation, but you cannot set `metadataObjectTypes` until the `AVCaptureMetaDataOutput` is associated with a session.

From the docs on `metadataObjectTypes`:

> When assigning a new array to this property, each of the type strings must be present in the array returned by the availableMetadataObjectTypes property; otherwise, the receiver raises an NSException.

and from the docs on `availableMetadataObjectTypes`:

> The available types are dependent on the capabilities of the AVCaptureInputPort to which the receiver’s connection is attached.

I was running the code as-is, and getting exceptions, so I found this SO answer http://stackoverflow.com/a/18995305/196358 which suggested adding the AVCaptureMetaDataOutput to a session before assigning the metadataObjectTypes. It solved my problems.