Add Developer Certificate of Origin Text

We purposely chose to license Docker under the Apache 2.0 license. This is a well-known
license, which is popular for its permissive and flexible properties, while still
encouraging a collaborative community. It also makes certain representations with regard
to contributions, and the rights given to contributors, the project, people who use Docker,
people who modify Docker, etc.

This approach to contributions is inspired by the popular Linux Developer
"Certificate of Origin". This approach makes it simple for new contributors to get started,
and avoids bureaucracy in tracking contributions and contributors.

To indicate accordance, each individual contribution to the Project is signed off by the
developer, using his or her real name, email address, and github handle in the format below:

	Docker-DCO-1.0-Signed-off-by: Joe Smith <joe.smith@email.com>
Merged with master
Return meaningful error for meaningless Dockerfiles
fix progressbar in docker push
add REMOTE_TODO
Remove old debug from tarsum
Make sure the cache lookup returns always the same result
Log files name along with their checksum in TarSum + add a Method to retrieve the checksum map
Change BuildFile in order to use TarSum instead of custom checksum
Hash the sums for directory (ureadable when there is too many
Replace FPM --vendor with --license, and give it the proper value of "Apache-2.0"

Fixes #3372
Make blank -H option default to the same as no -H was sent
Fix "docker load" help by removing "SOURCE" prompt and mentioning STDIN

Fixes #3383
Add more TestBuild Dockerfiles, especially for making sure manual RUN echo '#!...' continues to work correctly
simplify the volumes documentation and mention more details
Update for Ubuntu 13.10

With two additional commands, this procedure will work for Ubuntu 13.10 using the image stackbrew/ubuntu:13.10.

1) change /etc/pam.d/sshd, pam_loginuid line 'required' to 'optional'
2) echo LANG=\"en_US.UTF-8\" > /etc/default/locale
Mention lxc-checkconfig

This is important because the list below is not complete when one build one's own kernel.  Plus it is more robust in the face of changes.
Implement suggestions
Mac OSX command for pip was incorrect
Merge pull request #3374 from tianon/fpm-license-vendor

Replace FPM --vendor with --license, and give it the proper value of "Apache-2.0"