schemas/py_functions.sql: _namePart(): Fixed bug where it was returning the empty string instead of NULL

sql_io.py: import_csv(): Documented that sql.truncate() MUST be run so that the rows will be stored in inserted order, and the row_num added after import will match up with the CSV's row order

sql.py: add_row_num(): Add distinguishing comment to ADD COLUMN statement so that it will be cached. The distinguishing comment is required because sometimes column names are truncated, leading to unwanted collisions with previously-cached ADD COLUMN statements. It provides a way of distinguishing the full column name behind a particular ADD COLUMN statement.

sql_io.py: import_csv(): Free memory used by deleted rows from any failed import. Documented that sql.create_table() is not rolled back if the import fails, but instead is cached, and will not be re-run if the import is retried.

sql_io.py: import_csv(): Fixed bug where the added row number column needed to be named row_num instead of _row_num to be autodetected as the pkey column (sql.pkey_col) by sql.pkey() and to avoid name collisions with the row number column added in column-based import

sql.py: add_row_num(): Support custom row number column name

csv2db: Use new sql_io.import_csv()

sql_io.py: Added import_csv()

csv2db: Don't truncate the table before loading rows because it has just been created, and is therefore empty. This statement may be left over from a time when the table was created only once, and its creation was not rolled back if the import fails.

sql_io.py: cleanup_table(): Print 'Cleaning up table' log message