Remove length argument and init reading in constructor.

Todo: see if we can reliable move the initial file read
to the internal functions, e.g. by having a is_resource
check in all of them.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5525 2fea6a4d-a838-0410-917b-93a53c48e9d2