osx: Add Apple Remote support

After killing the non functional AR support in c8fd9e5 I got much complaints so
this adds AR support back in (and it works). I am using the HIDRemote class by
Felix Schwarz and that part of the code is under the BSD license. I slightly
modified it replacing [NSApplication sharedApplication] with NSApp. The code
of the class is quite complex (probably because it had to deal with all the
edge cases with IOKit) but it works nicely as a black box.

In a later commit I'll remove the deprecation warnings caused by HIDRemote's
usage of Gestalt.

Check out `etc/input.conf` for the default bindings.