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Allow calibre to be used by multiple users on Mac OS X.

Fixes bug #1320347

For reference see https://developer.apple.com/library/ios/documentation/General/Reference/InfoPlistKeyReference/Articles/LaunchServicesKeys.html#//apple_ref/doc/uid/20001431-113300
OS X Build: Fix a bug that was preventing multiple instances of calibre being used by different user accounts on the same computer. Fixes #1320347 [Multiple users cannot run Calibre concurrently](https://bugs.launchpad.net/calibre/+bug/1320347)

Merge branch 'fanf/multiuser' of https://github.com/fanf2/calibre