Accept arbitrary width integers in nunpack

[dhdaines]

Pull request

Integers in XRef streams, among other things perhaps, do not have to be 1, 2, 4, or 8 bytes. This fixes #886.

How Has This Been Tested?

There is a regression test using the document from #886 (thanks!).

Checklist

• [x ] I have read CONTRIBUTING.md.
• [x ] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x ] I have tested that this fix is effective or that this feature works.
• [x ] I have added docstrings to newly created methods and classes.
• [x ] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.