Fix for building in Xcode 5, use performSelector for UIDevice uniqueIdentifier.

This code no longer builds when using Xcode 5 because BPXLUUIDHandler calls UIDevice's -uniqueIdentifier method, which no longer exists. This change just only uses uniqueIdentifier if it is available. Note that this code only applies to the simulator, so for newer simulators it will generate a new UUID same as real devices.

![screen shot 2013-07-10 at 11 58 07 am copy](https://f.cloud.github.com/assets/631074/777502/38e75482-e99a-11e2-8ad4-cb384de526f1.png)