fix(package): import and "go get" as docopt-go

After renaming the **docopt.go** GitHub project to **docopt-go**
and back again, the redirect remains. So it is now possible to
`go get github.com/docopt/docopt-go`.

This updates all imports and other references to the downloadable
Go package to **docopt-go** while keeping the GitHub project name as
**docopt.go**.

Fixes #2.