make DBCreator class implement fluent interface

[lsloan]

For convenience, make the DBCreator class implement a fluent interface. In its most basic form, this means all the class' methods that currently return None would instead return the instance of DBCreator (i.e., self).

Such an interface would allow code like this:

    db_creator_obj = DBCreator(INVENTORY_DB, APPEND_TABLE_NAMES)
    db_creator_obj.set_up()
    db_creator_obj.drop_records()
    db_creator_obj.tear_down()

To be written as this instead:

    db_creator_obj = (DBCreator(INVENTORY_DB, APPEND_TABLE_NAMES)
        .set_up().drop_records().tear_down())

[ssciolla]

Makes sense. I'll try to handle this with some other changes I've been looking at to this class.