Add `script/bootstrap` to install dependencies in `./_vendor`

This helps newcomers who might have cloned the repo into an environment where they haven't yet set up proper GOPATH structure, and enables them to run tests and hack on the project.

It is also a step towards vendoring all dependencies in git; hence the cleanup of unnecessary files. Ideally no bootstraping should be required when someone clones this project.

/cc @jingweno