Add an http_multi_server package.

This package supports using the same server class for multiple bound interfaces.
In particular, it's useful for binding both IPv4 and IPv6 loopback interfaces.

R=rnystrom@google.com
BUG=19147

Review URL: https://codereview.chromium.org//311233004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@37023 260f80e4-7a28-3924-810f-c04153c831b5