refactor, remove useless buffers

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Fix Travis for PRs against non-master (like release, for example)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Fixes Issue# 3450
Docker-DCO-1.1-Signed-off-by: Gopikannan Venugopalsamy <gopikannan.venugopalsamy@gmail.com> (github: vgeta)
Add MAINTAINER and remove docker/utils dep from pkg/sysinfo

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Merge pull request #3652 from tianon/fix-travis-on-non-master-prs

Fix Travis for PRs against non-master (like release, for example)
Update Travis to also test compiling the docs

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3656 from tianon/travis-test-docs

Update Travis to also test compiling the docs
Merge pull request #3649 from vieux/remove_buffers

refactor, remove useless buffers
Merge pull request #3678 from creack/sysinfo_update

Add MAINTAINER and remove docker/utils dep from pkg/sysinfo
Merge pull request #3295 from alexlarsson/implement-untar

Implement UnTar via archive/tar
Extract mount into pkg.

Mount is self contained and generic, it should be in pkg, to allow other pkg modules to use it.

Docker-DCO-1.1-Signed-off-by: Paul Nasrat <pnasrat@gmail.com> (github: pnasrat)
Merge pull request #3680 from pnasrat/extract-mount-pkg

Extract mount into pkg.
Merge pull request #3292 from alexlarsson/export-changes-manual

Don't shell out to tar for ExportChanges
Merge pull request #3665 from vgeta/3450-fix

Fixes Issue #3450
archive: Remove unused features

This simplifies that code that calls out to tar by removing support
for now unused features.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
archive: Extract code from ExportChanges to addTarFile()

This is the code that takes a normal file and adds it to a TarWriter.
We extract it so that we can share it with Tar().

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Simplify addTarFile

We can use tar.FileInfoHeader to do much of the work.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Implement TarFilter in go, rather than calling out to tar

This uses a plain filepath.Walk + addTarFile to create a tar file,
optionially compressing it with gzip.

Unfortunately go only has gzip compression support, not bzip2 or xz.
However, this is not a regression, as docker currently uses *no*
compression for TarFilter(). The only compression of tarfiles
currently happens in utils/tarsum.go, and that manually does gzip
compression.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)