Resolve ARC potential leaks

Hello, thanks for a great library!

This pull request resolves some issues as reported by Analyze in Xcode 4.6 under iOS 6:

- Removed an unused dead store (into the `seconds` variable)
- Resolved potential leaks in `QWebViewController` by converting `CGImageRef` generators to return `UIImage`. There remains one more potential leak in this file that's not fixed in this pull, in `createContext`, with the `context` variable.