encode chapter related ids from str to bytes

[zvodd]

Don't pull. This was my work around but may offer some insight into a pain point.
But I don't think it solves the underlying issues.

When trying to run examples/chapters.py I was hitting errors like TypeError: can only concatenate str (not "bytes") to str and the like. I think the real issue is maybe the example is out dated and need to be passing in byte strings instead of strings.
or that the utils/requireBytes decorator should be applied more thoroughly to the ChapterFrame and TocFrame constructors.

[nicfit]

Hi @zvodd , thanks for the patches. Not sure if you checked out the chapters branch but I'd be curious whether the same problems occur there. The changes can be seen in #531 . And any chapter PRs should target that branch.