initial mods to extend class epub201

A start…

1. brought sanitize.php more in line with PB sanitize.php, so that we can use the proper call to canonicalize_url
2. class epub3 now extends epub201
3. many redundant functions could therefore be deleted, although I'd like to see more go 
4. modified epub201 class to accept a variable $filext (to allow switching btwn html/xhtml)