Fix for #119

As described in #119, there are cases where you have a set of columns that need to appear on multiple models. This change enables you to create a base class that inherits from `com.activeandroid.Model` that includes the set of columns that are common to multiple models. Then any model that needs to have the common set of columns can simply subclass that base class. (Also, make the base class _abstract_ if you don't want a table generated in your schema for the base class.)