Framework style headers for the Mac version

This branch will need a bit of cleanup, but I thought I'd open a push request to discuss it first, in case you're against the idea.

What I've done is created a ZipKit.h file, and made it and the other headers public.

For the Framework target this has the effect of allowing you to just to #import <ZipKit/ZipKit.h> to use it.

For the static library target, I've made the headers build into include/ZipKit, in the build products directory. This should also allow clients of the library to just to #import <ZipKit/ZipKit.h>.

In both cases it shouldn't be necessary to mess with include or framework search paths any more. As long as the relevant target (framework or static lib) is built before the thing that uses it, doing #import <ZipKit/ZipKit.h> should be enough.

The cleanup that I think should be done before pulling this branch is to change back to private all the headers that *don''t* need to be be public, and to change ZipKit.h to just include the public ones. I wasn't sure quite which ones should be public, so I've probably overdone it currently.