Added an Xcode project with a unit-test bundle target.

Includes Warnings.xcconfig in a submodule.
Import Foundation.h rather than Cocoa.h. If we need anything from AppKit in this class, we're Doing It Rong.
Moved ivars to implementation, replaced accessors with properties, modernized types.
Deleted commented-out lines.
Declare the constructors' return types as instancetype.
Removed use of autorelease. PRHLineEnumerator now requires ARC.
Removed unused variable that the analyzer didn't like. Build now succeeds.
Code style in empty prefab test class. (Super important.)
Implemented a basic suite of unit tests for various permutations of inputs.
Set the scanner's charactersToBeSkipped to an empty character set, so that we can scan newlines.
Scan any line terminator, not only \n.
Added tests for enumerating empty strings and strings containing blank lines.
Handle all known line terminators, and scan only one terminator at a time to improve blank-line handling.
Fixed blank-line handling: If we scanned a line terminator, then we have scanned a blank line and need to yield an empty (or terminator-only) string.