Modernization

Thank you for DDExpandableButton. We are using it in an upcoming project. As part of transitioning to iOS 6 + 7 only, I took some time this afternoon to modernize the code to require ARC, use properties more directly, auto-synth the ivars. I also cleaned up some code where it didn't read as well as I wanted. 

Next up is converting to Auto Layout so it will scale a bit better on 7.0 and forward.

I'm submitting the pull request, but if you're interested in supporting 3.0+ feel free to close it out and we'll continue our fork separately. 

Thanks!