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
Check standalone header when pinging a registry server. Standalone has to be true to use basic auth (in addition to previous requirements)
Fix #3149: port mapping of TCP + UDP

Signed-off-by: Tzu-Jung Lee <roylee17@gmail.com>
Fixed registry unit tests
Fix dotCloud to Docker, Inc
Remove the @github handle as it generates emails.
Add Travis CI configuration to validate DCO and gofmt

After each push, Travis CI will trigger, and check two things:

- make sure that each commit in the push has the Docker certificate of origin
- make sure that all .go files changed by this sequence of commits are correctly formatted in the most recent commit

Note: there is one edge case; if you do a git force push, we cannot figure out the actual commits in the force push, and we will just run the checks as if upstream master were the base. Pull requests will always be tested correctly, though.

Docker-DCO-1.0-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3435 from roylee17/3149-fix_port_mapping_of_tcp_plus_udp

Fix #3149: port mapping of TCP + UDP
Merge pull request #3475 from tianon/travis

Add Travis CI configuration to validate DCO and gofmt
Merge pull request #3431 from shin-/ping_standalone_check

Check standalone header when pinging a registry server.
Merge pull request #2091 from keeb/add-dco

Add Developer Certificate of Origin Text