Added iOS support.

Added a target for building an iOS static library and added a conditional include to `DAVRequests.m` because CoreServices is called MobileCoreServices on iOS.

I also took the liberty of removing unused frameworks and switching Cocoa for Foundation on the OSX target since it doesn't really need Cocoa expect for it's dependencies (like CoreServices) and those are better expressed explicitly so it works on iOS too.

The tests don't work because `DAVTestSettings.h` cannot be found, but that's something missing in `master` as well, so I left it as it was.