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.
Can you clarify this sentence?  I don't really follow what it is asking me to do, what is the "correct place under a directory ..."