API v3

This implements #54, therefore renaming `hoedown_markdown*` to `hoedown_document*` and renaming `markdown.{c,h}` to `document.{c,h}`.

This also creates the need for having `hoedown_version` in a separate pair.

While doing this, I also realized `hoedown_document_render` doesn't accept the `hoedown_document` instance as first argument, and corrected that for consistency with the rest of the API.

Build files and examples were modified accordingly.