Add rudimentary support for use on OS X.

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.
Make supporting multiple platforms (D-Bus and OSA Script) slightly more manageable.
Drop flet statement to avoid compilation warning on emacs 24.3.
Bjarte didn't contribute until 2013.
Avoid compilation warnings when D-Bus functions not available.
Added autoloading for main functions.
Merge pull request #2 from remvee/multi-platform

Multi platform; Add OS X support.