Create settings model and cell model

Created a model for the Settings data stored in the plist. Also created an abstract class for the table view cells so cell subclasses can be exposed and manipulated through one class
Not sure how the `versionNumber` could ever be set to anything other than nil prior to the `init` method. I would suggest getting rid of the property and public declaration of the `init..` method, and just overriding this method internally to call the super init method with `UITableViewCellStyleValue1` so the version number (detail text label) is always on the right, and just pull the version number from the info.plist automatically.