DQSqliteStatement : Added describe() function for describe the schema of a table.
DQBackendEngine: Added describeModel() function to provide information of a model in database.
DQSqliteStatement: Added alterTable() function to construct SQL of 'alter table'
Supports 'ALTER TABLE'
DQSharedQuery: Added finish() function to instruct database driver that no more data will be fetched form this query. Make sure the database is not locked.