Add explicit AppKit dependency for obs/platform-osx
Require cmake 2.8.12 on osx

This requirement is due to rpath usage in the project and SDK location
changes[1] in more recent osx versions

[1] i.e. http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=1fce189e
Update osx bundle files

The bundle icon now includes all recommended image resolutions and OBS
is now started as high resoltion application for high dpi displays,
still need to add high resolution images for icons used inside OBS