U4-4249 - PetaPocoExtensions CreateTable doesn't recreate the table

Amends `PetaPocoExtensions.CreateTable`, so if `overwrite` is enabled, the table is dropped, then recreated.

http://issues.umbraco.org/issue/U4-4249

Previously when the table was dropped, the value for `tableExist` would still be `true`, and was not recreated.