Fixed compiler warnings

Hi Håvard,

I fixed some compiler warnings that appear when building AGWindowView with -Wall. To reproduce:
- Use the Xcode project that comes with AGWindowView
- Add "-Wall" to Other Warning Flags (Targets > Build Settings > WARNING_CFLAGS)
- Compiler emits warnings "Pragma diagnostic pop could not pop, no matching push"

Please consider my changes for merging into your repo. Thanks for this project.

-Claus