Limit range of identifiers

[kleihan]

Fixes #265

[patricklucas]

As discussed offline, the issue appears to be that, due to now restricting IDs to the range of a Java int, the JAXB codegen now uses int instead of BigInteger for ID properties in generated classes.

The two solutions are to either configure the codegen to continue to use BigInteger, or to update the two classes in this repo that expect BigInteger to expect int instead.

I don't have insight into how many users this library has who might be affected by the change to int. However, we are already making breaking changes to the schema (and thus the codegen) which would require users to make code changes, so I don't think the change to int adds a significant burden beyond that.

@kleihan I have a commit prepared to change BigInteger references to int that I can add to this PR if you wish.