Add a password modify extended operation as specified in rfc 3062.

Also add two newa Error Codes (ErrorUnexpectedMessage and
ErrorUnexpectedResponse) that should be used (future patch) also in other
functions the by now logs an error instead of returning it.
Add an example of some password modify operation usages.
Merge pull request #2 from sgotti/passwordmodifyexop

Add a password modify extended operation as specified in rfc 3062 and an example.