Bug in handling of `0` ints

Hi,

I believe there is a bug on this line:
https://github.com/nexB/saneyaml/blob/40e5fa7c0b6e0012452053839184e5cd29802063/src/saneyaml.py#L284

It will cause an integer of "0" to be quoted. I think this would fix the issue:

`if len(value.lstrip('0')) == 0 or value.lstrip('0') == value:`

Or perhaps you just want to explicitly check for "0"