nREPL support

Hi James - as discussed, have added in nREPL support to the ```lein ring server``` etc commands. 

Have updated the docs, and verified that the code is correct with:

```clojure
(server-task (-> my-project
                 (assoc-in [:ring :start-repl?] true)
                 (assoc-in [:ring :repl-port] 7000))
             {})
```

where ```my-project``` was a project.clj map that I pulled from another project with leinjacker.

Let me know if this is the right place to put this functionality, or if there's any other changes you'd like me to make!

James