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.

* commit '3bbb1be7':
  Version 0.6.0
  Gotta commit those moved files
  Develomentmode ended up the wrong way around
  Move test helpers into seperate package
  Prevent package testing from going into the build

Develomentmode ended up the wrong way around



See merge request !29

Refactor upstream

Rationale: the code has become a tangled mess of global variables and
types that hang together when they need not. For example: every HTTP
handler uses a 'gitRequest'?? I want to clean this up and see if I can
move some things into internal packages.

Apart from using internal packages we now use http.Handler where we can,
and fewer global variables.

See merge request !20

We need to use `$(shell find` to find files from all subdirectories

Conflicts:
	main.go

remove make warning

When building gitlab-workhose a warning is printed out. This merge request fixes the warning

```
go build -ldflags "-X main.Version 0.5.1-20160103.233349" -o gitlab-workhorse
# _/home/git/gitlab-workhorse
link: warning: option -X main.Version 0.5.1-20160103.233349 may not work in future releases; use -X main.Version=0.5.1-20160103.233349
```

See merge request !22

Fix /api/v3/projects routing error

Non-special API requests were getting special treatment, which
resulted in 500 errors. This change avoids the special treatment and
adds tests that assert that regular API requests should be left alone
by gitlab-workhorse.

See merge request !26

We should test with what actually happens, e.g. foo%2Fbar, not
foo/bar.