Index: sort German umlauts correctly by including umlaut.ist.

For this to work properly all \index{} entries must escape umlauts with quotes, e.g. ``h"oflich``.
Add a ``quoted_umlauts`` method to the converter.

The method converts all umlauts to the quoted notation.
Merge pull request #36 from 4teamwork/jone-umlaut-in-index

Index: sort German umlauts correctly by including umlaut.ist.