- the "foreign_key" attribute on Column and ColumnElement in general

- the "foreign_key" attribute on Column and ColumnElement in general
    is deprecated, in favor of the "foreign_keys" list/set-based attribute,
    which takes into account multiple foreign keys on one column.
    "foreign_key" will return the first element in the "foreign_keys" list/set
    or None if the list is empty.
- added a user test to the relationships test, testing various new things this
change allows