uses NSPerformService() instead of calling -[NSWorkspace openURL:] to allow Quick Help to work without active network interfaces

OS: 10.8.3 12D76
Prevent the Dash not installed alert if the search string length is 0
Prevent Dash not installed alert in didClickOnTemporaryLinkAtCharacterIndex as well
Automagically detect the search platform (iOS or Mac)

The plugin will now check the active Xcode scheme and based on that
append a search keyword to the query.

Results: Dash will only search the iOS docset when you have an iOS
Scheme active or the Mac docset when you have a Mac Scheme active.
Update README.md
Update README.md
Merge pull request #5 from Kapeli/master

Automagically detect the search platform (iOS or Mac)