Add Vagrant links

cc: @jacobvosmaer-gitlab 

See merge request !175

More documentation cross-links



See merge request !172

Add 'gdk' command wrapper and gem

Combine `make` and `./run` into `gdk`. Can be invoked as `gdk`, e.g. `gdk run db`.

This does not remove `make` and `./run`, it just adds an integrated wrapper.

Installation become:

```
gem install gitlab-development-kit
gdk init # clones repo
cd gitlab-development-kit
gdk install gitlab_repo=https://bla
```

See merge request !174

Use a GOPATH to install gitlab-workhorse

This allows you to use more Go tools during development.

See merge request !173

This allows you to use more Go tools during development.

Reorganize the README

Fixes https://gitlab.com/gitlab-org/gitlab-development-kit/issues/126

See merge request !168

Merge branch 'master' of https://gitlab.com/gitlab-org/gitlab-development-kit into reorganize-readme

 Conflicts:
	Makefile
	README.md

Add .koding.yml for testing GitLab on Koding

ref: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4769

See merge request !170