Add support for Xcode image assets.

To use it, just pass in the path to Images.xcassets/AppIcon.appiconset
as the 2nd parameter. The script will check if it ends in .appiconset
and behave accordingly.
Combine version from CFBundleShortVersion and CFBundleVersion

Our builds increment the CFBundleVersion for each build. For betas
it helps us to see version numbers like "1.0 [22]" in the icon, with
1.0 being the short version and 22 the bundle version counter.

Also modified the automator call to handle the spaces in the version
variable correctly.