Add OS X support.

I already had a minor mode for this at https://gist.github.com/ljos/964611 , but saw that you where a bit further along and already in melpa. I have added the interesting bits from my mode.

You should check that it still works on linux as I didn't have the opportunity.
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Change spotify-debus-call into macro so we can decide if we need to
call applescript or dbus in compile-time.

Change name of spotify-debus-call to spotify-call to reflect the fact
that it now can be called on OS X as well.

Decide on eval if defuns and other things are needed by checking for
platform.