[zedai-to-html] Support pagebreaks in lists

 * pagebreaks placed before any list items are moved before the list
 * pagebreaks placed after any list items are moved after the list
 * pagebreaks between list items are inserted in the following item

Added XSpec tests.

Fixes [issue 356](https://code.google.com/p/daisy-pipeline/issues/detail?id=356)