PERF: speed-up DateFrame.itertuples() with namedtuples
- use [namedtuple._make()](https://docs.python.org/2/library/collections.html#collections.somenamedtuple._make)

Also:
- replace bare `except:` to avoid catching `SystemExit` and `KeyboardInterrupt`
- remove the generator `return` from the `try`-clause to an `else`
- more explicit fallback to regular tuples when `name=None` (docs?)
