download_file(): evaluate REMOTE_TIMEOUT correctly

In `download_file()`, the `REMOTE_TIMEOUT` configuration item was being evaluted at module load-time rather than when the function was called. The result was that changing REMOTE_TIMEOUT had no effect on timeout in `download_file()`. 

Aside: it looks like `timeout` is not being used in `download_files_in_parallel()`.

Probably should add this to `CHANGES.rst` as well...