Use gb to manage project and vendoring

[guilhem]

http://getgb.io/

[guilhem]

This is really useful to build project with good dependencies.

I know this is quite a huge PR... but it's only some moving and a gb vendor fetch for all dependencies I found :)

[guilhem]

We can also only commit vendor/manifest and use gb vendor restore to recreate local vendoring, but we are losing change from a lib to another.
Let me know what do you prefer

[emmaly]

I need to get more familiar with gb before pulling this. But I certainly like the idea. Give me a bit to review this.

[guilhem]

@dustywilson gb is doing vendoring for go project.
With this software we can specify a version for each library.
This prevent to fail if master on a lib has some breaking changes.

To use it, nothing more simple: install it with go get github.com/constabulary/gb/...
or by taking a precompiled one.

If vendor isn't populate (like #20 ), you have to restore lib with gb vendor restore. This will read vendor/manifest for version.
To build: gb build and.... that's all!

You can update a lib with gb vendor update $this_lib

You don't need to set $GOPATH or anything else, gb do all what you want :)

[guilhem]

After some use, I think I will remove all vendor/src and let people do gb vendor restore like in #20 .

We can also put some delivry like here: https://github.com/themecloud/etcd2envfile/blob/master/.travis.yml
to do pages like that: https://github.com/themecloud/etcd2envfile/releases