add prefer-timestamptz. fix adding-foreign-key-constraint. simplify prefer-robust-stmts

Added

• added prefer-timestamptz rule to warn about using timestamp instead of timestamptz. (#230)

Changed

• only apply prefer-robust-stmts to files with more than one SQL statement. (#231)

Fixed

• catch another way to add foreign keys for adding-foreign-key-constraint. Thanks @adamrdavid! (#228)