Some support for simple cases of upgrades

[BoD]

For instance, an attribute "addedInVersion" could be added on tables, and the tool would generate CREATE TABLE statements when upgrading from version n-1 to n. Another one "deletedInVersion", to delete a no longer used table (a less useful feature, but why not). I guess it may also be possible to handle table rename cases.

And also columns creation/deletion/renames, maybe.