Made ID string slightly larger

Pete pointed out in issue #14 that we have put an upper bound on the file format
versioning of 10 versions.  While that seems like plenty at the moment, it
wouldn't hurt to pad the version number to allow 99 versions.  I guess Pete
is either more optimistic than I am about how long this format will be useful
or more pessimistic in our ability to get things right the first time.

Either way, we have the necessary flexbility now.

This closes #14.