Use built-in model compiler instead of relying on Xcode

This updates mogenerator to use an internal model compilation scheme based on [momcom](https://github.com/atomicbird/momcom) whenever the model uses the Xcode 4+ file format. Model files are compiled directly in mogenerator instead of calling out to Xcode's compiler. This removes the dependency on Xcode's shifting location of the `momc` tool for those files.

Tests in `mogenerator/tests` pass.