Add minimalist .xcodeproj so rnpm finds DeviceUtil.{h,m}

[peritus]

Fixes #23

[dannolan]

+1

[GertjanReynaert]

@peritus Is this pr still valid for the current React Native version?

[peritus]

@GertjanReynaert I have no idea. I can rebase and test, there shouldn't be any dependencies on the react-native version, if you ask me.

[GertjanReynaert]

@peritus I'm just asking because I'm not that familiar anymore with React Native.
Since adding RNPM support is a pretty frequently requested feature, and this would enable RNPM, I would like to get this merged.

So if you could just validate that this works, then I'll merge it.