Add a tool to dump identifier constants from XIBs

[tonyarnold]

Closes #1.

[puls]

This appears to support old Xcode 4-style XIBs. I think it would be more worthwhile to have a XIB dumper either operate as a second file type in the storyboard dumper or handle both types.

(Also, keep #8 in mind, which will introduce drastic changes to the way these tools work.)

[tonyarnold]

These were XIBs created using Xcode 5.1 with a target of OS X 10.9 — have you got examples of a newer XIB format?

I'll take a look at #8, thanks!

[puls]

Nope, I was misreading your diff. Carry on, but again, I believe these are roughly the same format as storyboards, and it might be worthwhile to just extend that tool.

(As for using these tools for Mac development, it would be awesome if you wanted to take a stab at making the other tools Mac-friendly, too.)

[tonyarnold]

@puls, I'm keen to add OS X support via a command-line flag, but there are a few rough edges to work around yet (inputFileExtension being a class method amongst them). What's the ETA of merging #8? I'm happy to do it once that's merged.