Improve the HTML5 XSLT upgrader

- allows `@longdesc` on `img`
- fix the ambiguous match for `meta/@content`
- only convert `a` to `area` when the content model is compatible
Support @longdesc and @aria-describedat in html-to-fileset

Note that original URIs nust now be annotated in the source HTML as `data-original-*`
where `*` is the name of the attribute holding the URI.

For example:

```
<img src="img.png" data-original-src="..."
     longdesc="desc.xml" data-original-longdesc="..."/>
```
Simplify original URIs in html-to-fileset

- resource URIs are assumed to be those of the original resources
- vastly simplifies the implementation
- the `px:html-to-fileset` step no longer uses a `fileset.in` input
  port
- if possible, a new attribute `kind` is added to `d:file` to record
  the type of resource (e.g. image, video, description, script, etc)
Merge pull request #53 from daisy/feature/image-desc

Simplify and improve html-to-fileset