Voided and Signed not props

* moved voided and signed sections out of statement properties list
* re-numbered sections as needed
* no other changes were made in this commit

As discussed here: https://github.com/adlnet/xAPI-Spec/issues/220 , voided was included in the properties list even though it's not a property. When I went in to fix it, I saw that signed statements had the same issue.

The diff is a bit of a mess, but I promise I didn't change anything except moving the sections and associated TOC & section reference updates, including 1 section reference that already had the wrong #