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
hack/PACKAGERS.md: libdevmapper
Factor cache-probing logic out of buildfile.commit() and CmdRun().
Record added-tree sha256 in buildfile.CmdAdd, probe and use cache.
Add testcases for ADD caching, closes #880.
Add self to AUTHORS.
Add CHANGELOG.md entry for ADD caching.
Always copy dockerinit locally, regardless of whether our docker binary is static, because even it might get deleted or moved/renamed
Add dockerinit SHA1 and path to "docker info" when debug mode is enabled
Allow packagers to specify a custom dockerinit lookup location via DOCKER_INITPATH in dynbinary

Only necessary if distro policy dictates that the path deviate from the paths already listed in utils/utils.go - please refrain from using it otherwise.
Handle compressed tars in ApplyLayer

When pulling from a registry we get a compressed tar archive, so
we need to wrap the stream in the right kind of compress reader.

Unfortunately go doesn't have an Xz decompression method, but I
don't think any docker layers use that atm anyway.
Tidy up some of the error messages from apt during build
archive: Re-add XZ compression support

This shells out to the xz binary to support .tar.xz layers, as
there is no compression/xz support in go.
Handle external mounts outside of lxc
Fix unmount issues
Improve interface by moving to subpkg

Enable builds on OSX
Move sqlite conn to graph db for cross compile support
Make Tianon the official root "Dockerfile" maintainer, since it's so hard-locked to hack changes most of the time
Use box-drawing characters in `docker images -tree`

This makes the output of `docker images -tree` look a little prettier.
Previously it displayed a combination of box-drawing characters and pipe
characters, so the lines didn't quite connect...

Before:

    └─aceb1e132fe5 Size: 487 MB (virtual 1.728 GB)
      |─c5480c55e00a Size: 44.89 MB (virtual 1.773 GB)
      | └─96c21b5e3c80 Size: 17.25 kB (virtual 1.773 GB)
      |   └─58f3f2293512 Size: 8.191 MB (virtual 1.782 GB)

After:

    └─aceb1e132fe5 Size: 487 MB (virtual 1.728 GB)
      ├─c5480c55e00a Size: 44.89 MB (virtual 1.773 GB)
      │ └─96c21b5e3c80 Size: 17.25 kB (virtual 1.773 GB)
      │   └─58f3f2293512 Size: 8.191 MB (virtual 1.782 GB)