Improve network performance for VirtualBox

Without those lines, VitrualBox machines are being very slow with all network operations (tested only in OS X Lion, but they are supposed to work everywhere). See https://github.com/mitchellh/vagrant/issues/1807
Split stdout stderr for non-tty commands
Add stderr unit test
Make StdCopy works with huge amount of data
Add documentation
Add proper bash completion for "docker push"
Container: Always create a new session for the container

We never want the container to be in the same process group as the
daemon, as then the container will receive signals sent to the
process group of the container.
Adding more tests around `auth.ResolveAuthConfig`

mostly because I've been failing at getting docker `0.6.2` working with
docker-registry `0.6.0` with regard to login and push.  I suspect there
may be some mismatched expectations between `auth.ResolveAuthConfig` and
`registry.ResolveRepositoryName`, but I haven't done enough research or
experimentation to think it's anything more than user error.  More tests
are good, no?
Fix #1993: add explanation for export restrictions
Replace deprecated upgrading reference to docker-latest.tgz, which hasn't been updated since 0.5.3
Update Gentoo installation documentation now that we're in the portage tree proper

We're officially a first-class Gentoo citizen now, which is very exciting.  Many thanks to @gregkh for helping us get here.

I started just adapting sections of language in this document, and realized several bits needed to just be rewritten entirely to be more clear.
Merge pull request #1989 from alexlarsson/setsid

Container: Always create a new session for the container
fix the error message so it is the same as the regex issue #1999
Merge pull request #2004 from kencochrane/master

fix the error message so it is the same as the regex issue #1999
Merge pull request #2001 from tianon/gentoo-tree

Update Gentoo installation documentation now that we're in the portage tree proper
Merge pull request #2000 from tianon/deprecated-docker-latest-tgz

Replace deprecated upgrading reference to docker-latest.tgz, which hasn't been updated since 0.5.3
Enable SSH Agent forwarding in Vagrant VM, so one is able to clone repos from private git repos etc.
Add the X-Docker-Registry header and /v1/_ping endpoint
Remove smart quotes. Add the X-Docker-Token header

Also fix the /layer endpoint, which had incorrect information
Fix Authorization headers