- [feature] The "extend_existing" flag on Table

- [feature] The "extend_existing" flag on Table
now allows for the reflection process to take
effect for a Table object that's already been
defined; when autoload=True and extend_existing=True
are both set, the full set of columns will be
reflected from the Table which will then
*overwrite* those columns already present,
rather than no activity occurring.  Columns that
are present directly in the autoload run
will be used as always, however.
[ticket:1410]