Changed the token lists for pre/post abstract into hooks.
Changed the "epub segments" counter into an L3 int and fixed the list
of files in the spine so that it gets all of them.

Fixed the rules for when to split sections into separate files.
Added the ASCII7 encoding for the time being
Wrote the README to give some basic instructions.

(Note that README.tex is the master file and README.md is generated
from it, but in order for it to be the README in github it has to be
versioned.)