A few fixes and enhancements for iOS and WP8

- iOS and WP8 now show the title if message is null and title was passed (on iOS it was absent, on WP8 it showed the text 'null')
- WP8: implemented tap to cancel (like on the other platforms)
- WP8: implemented invocation of cancel callback
- Slightly darker iOS background
- REAME update to reflect these changes

Enjoy,
Eddy