Follow Location header with relative URL

Fixes #19

I'm a little concerned about following relative paths, because follow is no more idempotent
```
http://localhost:3333/> get
*** Type fol[low] to follow the 'Location' header received in the response
 200  OK -- 4 headers -- 6-character body
http://localhost:3333/> headers
         Location: found
             Date: Fri, 14 Feb 2014 15:49:54 GMT
       Connection: keep-alive
Transfer-Encoding: chunked
http://localhost:3333/> follow
http://localhost:3333/found> follow
http://localhost:3333/found/found> follow
```

@njonsson what do you think?