Rewrite `script/build` and `script/version` in Go

A cross platform build script is needed to build `hub`. Instead of
maintaining different shell scripts for *nix and Windows, writing
everything in Go is a viable approach - all we need is a Go compiler.

The process of building hub becomes building make and using the make binary to build hub:

```sh
$ go run make/*.go . -b

or

$ go build -o hub-make make/*.go
$ ./hub-make . -b
```

This is a proof of concept. I'm taking feedback before I go further, e.g. `script/package` needs to be rewritten as well.

/cc @mislav