Fixes Xcode Quick Help → Dash not working when offline

Uses `NSPerformService()` instead of calling `-[NSWorkspace openURL:]` to allow Quick Help to work without active network interfaces.

**Tested on:** OS X 10.8.3 (12D76), Xcode 4.6 (4H127), Dash 1.8.0
**Reference:** https://github.com/omz/Dash-Plugin-for-Xcode/issues/2