more major changes for version 2.x

-- #141127 by leo pape and jpetso with modifications,  private files support + access control to derivatives.
-- #141127 by jpetso, added last modified header
-- #141127 by jpetso, removed Content-Length header per jpetso
-- #177902 by doug green, strip file_directory_path from urls and paths.

-- added default 403 and 404 image support (ftp only, no ui yet)..
  --  imagecache/$presetname.403.png
  --  imagecache/$presetname.404.png
-- 404 response if preset unknown.
-- 404 response if source image and 404 image not found
-- 403 response if access denied and 403 image not found.
-- 500 response on error instead of letting drupal try to render
a page.

-- code restructuring, more reusable functions.