fix status request

As I can see in Elasticsearch v1.3.1 status "/" page does not return "ok" field.

Also it will be nice to use "eitherDecode" instead of "decode" in getStatus function.

```haskell
getStatus :: Server -> IO (Maybe Status)
getStatus (Server server) = do
  request <- parseUrl $ joinPath [server]
  response <- withManager defaultManagerSettings $ httpLbs request
  return $ decode (responseBody response)
```
will be somehing like
```haskell
getStatus :: Server -> IO (Either String Status)
getStatus (Server server) = do
  request <- parseUrl $ joinPath [server]
  response <- withManager defaultManagerSettings $ httpLbs request
  return $ eitherDecode (responseBody response)
```


at https://github.com/bitemyapp/bloodhound/blob/master/Database/Bloodhound/Client.hs#L92