Prevent showing a dock icon and stealing focus when running.

[`NSApplication#setActivationPolicy:`](file:///Users/donovan/Library/Developer/Shared/Documentation/DocSets/com.apple.adc.documentation.AppleOSX10_8.CoreReference.docset/Contents/Resources/Documents/documentation/Cocoa/Reference/ApplicationKit/Classes/NSApplication_Class/Reference/Reference.html#//apple_ref/occ/instm/NSApplication/setActivationPolicy:) has been around since 10.6. I'm not entirely sure why the LSBackgroundOnly flag doesn't work.

I tested this on OS X 10.9 only.