Make sprout-osx-settings::remove_expose_keyboard_shortcuts recipe idempotent

After many slow chef runs, I got a bit fed up of the system preferences
select box pane popping up & the delays caused by this recipe.

I was able to find a way to automate these preferences a bit faster, and
also a way to skip the ruby blocks if not necessary.

References:

[Scriptable System Preferences][1]
[Reset FKey Script][2]
[Daring Fireball: How to Determine if a Certain App Is Running Using AppleScript and Perl][3]

[1]:
http://www.macosxautomation.com/applescript/features/system-prefs.html
[2]: http://askpascal.com/wiki/index.php/Reset_FKey_Script
[3]: http://daringfireball.net/2006/10/how_to_tell_if_an_app_is_running