This makes it easier for a package to replace an different package with itself
while it is being imported.

there is a CONTENT_LENGTH header, regardless of the request method.

the access control check that requires aq_inContextOf. Also fixed the
content-type being set as precondition problem for Files and Images, and
made *all* text PUTs create Documents, not just text/html PUTs.

in tree tags.

stuff. Also a couple misc tweaks. SystemExit is still not done.

sized input for STDIN collection. It's not perfect yet.

added ready medthod to producer interface and updated most existing producers
to work with it. The ready method allows future producers.