DAISY 2.02 Validator

This is a port of the "Daisy 2.02 DTB Light Validator" included in Pipeline 1. This implementation does not yet support validation of the MP3 files themself.

I've developed this on the same branch as the EPUB3 to DAISY 2.02 script, which is why the commit log contains some of those commits. However, this PR only contains the DAISY 2.02 Validator script and can be merged independently. The EPUB3 to DAISY 2.02 script will be in a separate PR.

<!---
@huboard:{"order":0.09375,"milestone_order":49,"custom_state":""}
-->

Yeah, I'm not sure it's an issue, I was just curious – but we have to keep in mind that it can be a performance bottleneck.

I don't know what's the biggest DAISY 2.02 production book, @marisademeglio may have  a better idea with her AMIS  experience. The biggest sample I have is the [Microsoft Frontpage 2003 manual](http://www.daisy.org/sample-content#t8), which is 500+ MB zipped, with 5+ MB of HTML, and 7+ MB of SMIL in 835 docs.

> Do you have any ideas for a pattern that might be more efficient?

Unless we do it in Java, or unless we start using an XML DB as an internal temporary storage system for big content, not really. Like you said, it might perform better to quickly compile a list of references and IDs (from Saxon), then working on that (from Calabash or Saxon). But I'm not convinced it's worth the work for now. Again, let's keep it as-is until we see real-world performance issues.