MKAnnotation extension forces linking to MapKit

[mxcl]

I'd rather not be forced to link to MapKit by a dependency, but especially I don't want the binary size increase due to the Swift MapKit compatibility layer that gets bundled with my app because I want to use YYWebImage.

If you agree, my suggestion would be to make it an opt-out subspec so that nothing changes for existing users, but those of us that care can not use it by specifying:

pod 'YYWebImage/Core'

I do this for PromiseKit.