Fix button background on touch

The customisation added in pull request #3 seems incomplete in its implementation of button backgrounds on a touch event. I have provided a complete implementation which seems to fit with the code that was already present. Since button backgrounds are not well supported by the iOS API, I've used the method described in [this stack overflow answer](http://stackoverflow.com/a/14525049).

I added the now working method to the 'simple customisation' alert view in the demo project for completeness too. Both methods work with the functionality demonstrated in the 'two button alert view'.

If this isn't the expected behaviour, I'm happy to fix it up since I require this kind of functionality and rolling it back in is no problem.