[Distutils] [Buildout] Download API and extends caching - please review

[Thomas Lotze]

As discussed before on this list, I've been working on an internal
download API for zc.buildout that takes care of handling the download
cache, offline mode, MD5 checks etc and is used for fetching configuration
files (as specified by the buildout:extends option). I consider the
feature finished and I'd be grateful if someone could review it before it
gets merged to the trunk. The branch I've been developing this on is
<svn://svn.zope.org/repos/main/zc.buildout/branches/tlotze-download-api>.
Thank you.

-- 
Thomas