revised HTTP-WG charter

Although the Milestone dates are certainly subject to discussion,
I thought it would be useful to have a revised charter where
the dates were somewhat in the future rather than quite a bit
in the past. So I've attempted to make some simple guesses
as to what I think are good targets, and suggest we use these
as a starting point for discussion.

It's also a good idea to update the charter's text to reflect
the reality of what we're working on now.

questions, comments, etc. welcome.

Larry