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.
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)
Move root symlink check to engine.New

Since commit c91c365, when starting the docker daemon without an
existing /var/lib/docker directory, it fails with:

  2013/12/18 23:39:36 Unable to canonicalize root (%!s(*string=0xc210077c80)): lstat /var/lib/docker: no such file or directory

Move the symlink checking code to engine.New after the root dir has been
created.
Merge pull request #3270 from vreon/tree-box-drawing-characters

Use box-drawing characters in `docker images -tree`
Merge pull request #3275 from crosbymichael/sqlite-import

Move sqlite conn to graph db for cross compile support
Merge pull request #3277 from jpoimboe/fix-root-symlink

Move root symlink check to engine.New
Merge pull request #3274 from tianon/tianon-dockerfile

Make Tianon the official root "Dockerfile" maintainer
Add new "cross" bundle to cross-compile the Docker client for other platforms (currently just 32-bit and 64-bit OS X)
Merge pull request #3276 from tianon/cross-compile

Add new "cross" bundle to cross-compile the Docker client
Merge pull request #3243 from alexlarsson/compressed-tar

Handle compressed tars in ApplyLayer
Merge pull request #3271 from crosbymichael/mount-outside

Perform docker specific mounts outside of lxc