Add ZXing 2.3

Hi, 
ZXing 2.2 would not build anymore on XCode 6. I am a former maintainer of ZXing so I created a separate repo for ZXing specific iOS code. As explained in [this readme](https://github.com/applidget/zxing-ios). 
The enhancement of this new version are:
- Now working with Xcode 6
- A lot faster to install since it does not clone the whole ZXing codebase and test suite