Avoid quoting `None` as a string in path segments.

Leading to all calls to `es.index` without an explicit id to use the same
id of `u'None'` and thus override the same document.
fix docstring to include special `None` case
Tweak '' comparison to be clearer. No behavior changes.
Update readme for 0.4.1.
Fix mistaken stringification of "None" in `index()` calls that omit `id`. Fix #90.