Added a GUI app that runs appledoc.

It's document-based. An “appleseed” file specifies all of the facts that appledoc requires. You should be able to keep these in version control and then just open the file and hit the button to do the work thereafter. They're JSON-based, although it'd probably make the app simpler to change them to plists (JSON doesn't support NSData objects, so this implementation Base64s them).

I have no immediate plans to add any other appledoc arguments to the GUI or the file format, but neither one should be difficult.

Known bugs:
- The source root URL is blank again when opening a seed file. Needs prompt investigation.
- No output/progress/activity display yet.

One obvious possible feature: A second button to generate a .xar archive. Might need a bit of internal reworking.

Likely never to see the App Store, since you can't have a document-scoped bookmark for a directory (such as SRCROOT), and I haven't figured out how document-scoped bookmarks can possibly be saved and restored in the NSDocument write and read methods at all (since self.fileURL is likely to be nil).