[epub3-nav-utils] Include the full nav outline

In `px:epub3-nav-create-toc`, only the first item of the HTML outline was included in the ToC. This is fine for some cases, but if the HTML outline has
several top-level entries (e.g. several `h1` elements), only the first outline
branch was included.

This is now fixed by this PR. Fixes [issue 352](https://code.google.com/p/daisy-pipeline/issues/detail?id=352)