[SQLObject] SQLObject inheritance

[Ian B. <ia...@co...>]

I made some changes, and it should now be possible for SQLObject classes
to inherit from each other.  The inheritance structure is not, however,
reflected in the database.  So:

class Person(SQLObject):
    name = StringCol(length=20)

class Employee(Person):
    salary = CurrencyCol()

The employee table is then expected to have both a name and salary
column, and references to Person objects cannot transparently reference
Employee instances (you have to reference one or the other).

I like the idea of subclasses (like Employee) sharing a primary key with
their superclass, and potentially the superclass having references as to
which subclass they belong to (from that modeling paper Luke referenced
a while ago).  But I'm not sure how best to actually implement that --
both in the database/SQL, and in the Python side.  So that's a future
goal, and wouldn't replace this current kind of inheritance (just
provide an alternative).

  Ian