Use godep to manage dependencies

This PR make hack/build-go.sh to use [godep](https://github.com/tools/godep) to manage dependencies. In fact, only few lines needs to be changed. I also added a script hack/update-deps.sh which could add/update a specified dependency. In this case, we don't need to change dep.sh manually because [godep](https://github.com/tools/godep) could analyze the source code and put dependencies under Godeps/ directory. It does exactly what we did, but smarter on many side. Besides, it is also a common tool to Go community.

For more details about benefit of using [godep](https://github.com/tools/godep), please checkout #376 

There's also some unused packages in our old third_party/ directory, which is removed in this PR.
Oh. Basically, it means that kubernetes should be in ``$GOPATH/src/github.com/GoogleCloudPlatform/kubernetes``. However, because we could put several directories' path in ``$GOPATH``, so the I used an awkward expression there. I think  ``$GOPATH/src/github.com/GoogleCloudPlatform/kubernetes`` would be fine.