Splat url helpers

Closes https://github.com/phoenixframework/phoenix/issues/527

@josevalim - Still couldn't figure out what to do with `NAME_path(conn | endpoint, ...args)`. At first I thought I'd use path_info in `%Conn{path_info: path_info}`, but it will only work for a conn that has the correct path_info. What in conn do I need to use for `NAME_path`?


In here you can actually just delegate to the `url/2` defined above. Something like this:

```elixir
def unquote(:"#{helper}_url")(conn_or_endpoint, unquote(action), unquote_splicing(vars), params) do
  url(conn_or_endpoint, unquote(:"#{helper}_path")(conn_or_endpoint, unquote(action), unquote_splicing(vars), params))
end
```

This way you don't need to pattern match on the connection or the endpoint!