[PATCH 0 of 2] support for faster auth in http (Testing wanted)

[Augie Fackler]

This patch series adds support for using Expect: 100-Continue to verify
authn before sending the entire bundle to the server. This should allow
us to push large bundles only once after we actually are authenticated.

The client side of this requires a completely new http library, which
is available at <http://bitbucket.org/durin42/async-http/>. The hg.py file
in that repository is an extension for Mercurial that patches the new code
into urlopen2 so that Mercurial will use it.

I'd really like some people to test this in a variety of situations and
contribute bug reports and code, as I know this is missing features we
support now. Notably missing right now is proxy support and SSL certificate
authentication. I'm not sure what else is missing or broken.