Do not manage old unicorn process in `unicorn:restart` 

Similar to #20 this removes killing the old unicorn process in `unicorn:restart`. Additionally this adds a bit of gem scaffolding stuff and makes the old behavior available via `unicorn:legacy_restart`

Suggested Changelog text:

```
Breaking change: `unicorn:restart` task no longer attempts to manage the old unicorn process since the prevailing
widsom is to manage that process via the `before_fork` block. If you were depending on this behavior, you may change
your `deploy.rb` to replace the `unicorn:restart` task with `unicorn:legacy_restart`. If you were not depending on
this behavior, no changes are necessary.
```