Hi,

the attached patch adds the following options to the scrlttr2 exporter:

- #+PLACE, or org-koma-letter-place: the place before the date,
  e.g., "Berlin, 28. Dezember 2012"; the place is only typeset if this
  variable is defined
- #+SUBJECT_TITLE, or org-koma-letter-subject-title: the subject keyword
  on the subject line, e.g., the German "Betreff"

It also fixes the behavior if no signature is defined, i.e., it uses the
KOMA variable fromname as described in the documentation.

Cheers,
Viktor