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 #367

There's also some unused packages in our old third_party/ directory, which is removed in this PR.
Oops. I did not notice that. Which directory should we put this script? Considering there's no third_party if we want to use godep, how about put it into a separate directory called htpasswd?