Issue #14 Added I25 Code

`AVMetadataObjectTypeInterleaved2of5Code` was introduced with iOS8. As this library is currently backwards compatible to iOS7 we need to make sure this constant exists before using it. This probably needs to be represented in `metadataObjectTypeFromScannerCode`. Could you add this to the PR? Otherwise I would merge this into the feature branch of issue #7 once I start with that.