Add Store.database_key column.

This seems hopefully useful, although the larger question of how to relate
database instances to stores etc. is not well answered yet.

rattail/db/model.py

@@ -370,6 +370,7 @@ class Store(Base):
     uuid = uuid_column()
     id = Column(String(length=10))
     name = Column(String(length=100))
+    database_key = sa.Column(sa.String(length=30))
 
     def __repr__(self):
         return "Store(uuid={0})".format(repr(self.uuid))