Added Xcode project. Generated libNUI.a to make use in projects easier.

I think it's a hassle to import all .m/.h files in my Xcode projects, I'd rather import a subproject into my main project and link against a library.

- Added Xcode project generating libNUI.a so it can be easily imported in another Xcode project.
- Added Xcode related files to .gitignore.
- Created NUI.h to import in one place all files a user may need when using NUI.