add an option for row versioning

[Mike Bayer]

this would be an option to mapper called "version_column", which references a column that stores an integer version number. SA can then retreive this version id and add it to the WHERE criterion when doing UPDATE and DELETE statements as part of the "check the row count" "optimistic concurrency" function.

[Mike Bayer]

working unit test for update/delete added in changeset:1253