__call__ now always returns a string value
- renamed resolve to values in HTTPHeader to better
  reflect the public-interface for this (esp for multi-entry headers)
- a few bugs in mult-entry headers
- added common CGI headers to httpheaders; I know they don't
  really belong here, but error checking is nice
- updated auth.digest and auth.basic to use httpheaders
  (this is what prompted the above changes)
- added WWW_AUTHENTICATe header which will build a response to
  a digest challenge
- fixed capitalization error in fileapp and added corresponding test

  they are quite private and should (usually) be singeltons.
- updated all instances to use UPPPER_CASE format, like CONTENT_TYPE
- propigated these changes to tests and to fileapp.py

- updated fileapp.py to reflect new header

btw... how do I assign bugs to myself?  I think I need a login.

- added abstract parse function to HTTPHeader

- update httpserver to not include HTTP_CONTENT_TYPE or HTTP_CONTENT_LENGTH
  header fields as discussed on Web-Sig

oh, last change had one other thing:
- update ContentType and ContentLength to use the CGI variables rather
  than the corresponding HTTP_ variables in environ

- renamed Expires.time to Expires.parse for consistency
- updated FileApp/DataApp to return 206 on Partial Content
- all HttpHeader(environ) return strings (empty string when not found)
  so that checks like 'if header-part in HttpHeader(collection)'
  works without having to check for None
- updated FileApp to use Range header (instead of having its own copy)

- fixed bug in content-disposition
- fixed bug in cache-control

next-up: Content-Range, Range headers