Add support for dc:type

[HadrienGardeur]

In EPUB 3.0.1, dc:type is tied to an EPUB controlled vocabulary that we should attempt to store in our in memory model and in the Web Publication Manifest.

Since prior to EPUB 3.0.1 other values could be used, it's probably best to filter this and only support controlled vocabularies.

[HadrienGardeur]

It's unclear at this point, how exactly this should be represented in the JSON, which is why this issue isn't in the backlog yet.

Since this is fairly specialized in EPUB, I'm tempted to group it with similar properties.

[HadrienGardeur]

So far, the following metadata seems very EPUB centric and not necessarily useful elsewhere:

• rendition properties
• EPUB type
• MO properties (active class, skippable elements)

These are likely candidate for an extension in the Readium Web Publication Manifest, rather than being first class citizens.