Sound and Music for RoboVM-based iOS backend using ObjectAL.

This adds Sound and Music support on iOS when using the RoboVM backend. The ObjectAL static library is assumed to be located in
```
gdx/libs/ios32/libObjectAL.a
```
as a fat library containing both arm and x86 versions of ObjectAL. This patch does not include code to build ObjectAL. Here's how I'm building it:

```
tar xvfz kstenerud-ObjectAL-for-iPhone-v2.2-0-gebd5a1e.tar.gz 
cd kstenerud-ObjectAL-for-iPhone-ebd5a1e/ObjectAL
xcodebuild -arch armv7 -sdk iphoneos
xcodebuild -arch i386 -sdk iphonesimulator
lipo build/Release-iphoneos/libObjectAL.a build/Release-iphonesimulator/libObjectAL.a -create -output build/libObjectAL.a
cp build/libObjectAL.a ~/Projects/libgdx/gdx/libs/ios32/
```

Note that this requires the latest RoboVM 0.0.4-SNAPSHOT nightly to be used. I will release a 0.0.4 version some time next week.