NULL value in Lua represented by odbc.NULL
Fix. Convert NULL values in luasql module.
All methods in dba module use error objects.
Fix. Query.fetch_all for opened query.
Add. Catalog functions can use as iterator or return data.
Add. Transaction manipulation.
Merge branch 'NULL' into dba
Add tests from lua-dba.
Add some function to support lua-dba interface.
Fix some tests.