Generate correct code for different databases in DoGenerateBackupTable

[Stefan Glienke]

Various databases support different ways of creating a table by providing a query. Unfortunately Firebird does not, see the comment in http://tracker.firebirdsql.org/browse/CORE-796 for a list of databases that support one or the other way.

This might also affect the method DoGenerateRestoreTable.

[Linas Naginionis]

fixes #49

→ <<cset e9f32fa0ca09>>