ctxhttp provides helper functions for performing context-aware HTTP
requests.

Fixes golang/go#11904

Change-Id: Ib9d2fef48953dbb52f0a70e1ed49ea7fe12b7801
Reviewed-on: https://go-review.googlesource.com/12755Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Aaron Jacobs <jacobsa@google.com>
Reviewed-by: Dave Day <djd@golang.org>