fix a compile error

When I must change my `AppDelegate.m` to a `AppDelegate.mm` file for some reason, I will get the following error:

![3dd886cf-6d3e-487c-a2ef-0231aa80475c](https://cloud.githubusercontent.com/assets/2641747/2993237/f93dc644-dcab-11e3-89b6-e2ecb090e3a9.png)

Is it better always compile it as c method?