Document the 'Snap.Test' module.

Thanks so much Mikhail, this looks much better already.

A few tweaks:

  * You can move the version to the GET line (i.e. you can render the http status line the same, `GET /foo/bar HTTP/1.1`)

  * "pthinfo" can be omitted (we maintain this identity:
 
```haskell      
rqURI r == rqContextPath r <> rqPathInfo r
               <> let q = rqQueryString r
                   in (if S.null q then "" else "?") <> q
```
 
   so the pathInfo can be deduced from the URI and the context.

  * Change "cntntlen" to "clen", and make `Nothing` render as "n/a"