Run ImageOptim in CLI mode instead of AppleScript

I can't find a good reason to run ImageOptim via AppleScript when [it supports being run from the command-line already](http://imageoptim.com/command-line.html). This patch does away with the AppleScript invocation and runs ImageOptim directly.

It should also be noted that ImageOptim produces quite verbose output when running in this mode which could be used to avoid doing the copy and replace action entirely.