Misc fixes: initialize.py confusion, --no-readline, sys.executable, IntegerDigits bug

This is a rollup of a few totally unconnected minor fixes. Some of these might be a bit controversial:

- Add a dummy `initialize.py` which explains how to use `python setup.py initialize`. This is just to be friendly to people who upgrade. I was thinking that maybe it could be included in 0.6 final, and then disappear in 0.7?
- Use sys.executable when printing the "database not found" error (to follow 464a2547592)
- Remove a rounding function which looked like it was unused
- Fix a bug where `IntegerDigits` could miscalculate the number of digits it needed to return