iOS: Use PDF instead of PNG

Instead of using PNG files, consider using PDF files with vectors. Xcode then compiles whichever sizes it needs (it renders the PDF into bitmap during compile time). This has several advantages:

a) less files.
b) won't include @3x on OS X, where it's not needed (yet).
c) were there @2.5x or @4x in the future, it will be supported out of the box.
