Add missing entries in build artifacts archive metadata

We need full directory structure, but since ZIP does not require it, we
need to calculate missing entries for directories when we generate
archive metadata.

Closes gitlab-org/gitlab-ce#12634

cc @jacobvosmaer @ayufan 

See merge request !34

We need full directory structure, but since ZIP does not require it, we
need to calculate missing entries for directories when we generate
archive metadata.

Use Go 1.5.3



See merge request !33

Add DESTDIR support to "make install"

Fixes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/8

See merge request !27

Use gitlab-zip-cat to send zip entries

Fixes https://gitlab.com/gitlab-org/gitlab-workhorse/issues/17

See merge request !31

Configure URLPrefix before building routes

Fixes #19 

See merge request !32

Add support for new artifacts features

It is proof of concept implementation that misses following:

- [x] Saving metadata to file, and passing that file to rails,
- [x] Making it look nice,
- [x] Have `refactor-upstream` merged: https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/20
- [x] Add tests for metadata and single file downloads,
- [ ] Spawning separate process to verify zip archive **ONHOLD**: https://gitlab.com/gitlab-org/gitlab-workhorse/issues/17,
- [ ] Spawning separate process to download file from zip archive **ONHOLD**: https://gitlab.com/gitlab-org/gitlab-workhorse/issues/17


See merge request !28

This test duplicates some of what is in internal/artifacts but
@grzesiek and I felt it was important to have an integration test that
also covers the route to this resource.

Uses escaper from mime/multipart/writer.go.