Add xcodeproj to build PSPDFAlertView and PSPDFActionSheet as a static library

The attached xcodeproj has two targets: libPSPDFAlertView.a and libPSPDFAlertView+arm64.a. The former links against the iOS 5 SDK and produces a binary that supports armv7 and armv7s, while the latter links against the iOS 6 SDK and produces a binary that supports armv7, armv7s and arm64.

Also: #import UIKit in UIColor+PSPDFKitAdditions.h to work without requiring UIKit to be #import'ed/@import'ed elsewhere.