Build documentation using appledoc

This fixes #28 by adding a script file that generates an Xcode docset for all files under `Classes`.

I decided arbitrary values for the appledoc configuration (eg. --docset-feed-url/package-url).

This also updates GTReference.h to use the appledoc comment syntax.

Note: appledoc still doesn't support some *obvious* things, like enum, blocks and typedefs.