Move cell init into a common setup

Related to #11: the disclosure indicator wasn't being shown because the `init` method was never called; `initWithStyle:reuseIdentifier` was. I moved the init logic into a common setup method and call that from all four init methods.

@vokalinteractive/ios-developers anybody around for a quickie?
I guess NSCoding just hasn't been updated to the `instancetype` standard yet.