Make stemming language configurable

This change allows you to configure the language used for stemming,
which should result in better search results for users with
predominantly non-English mail.

This change is backwards-compatible - the default stemming language
remains English, unless a specific language is set in your config.yaml.
To do so, add the key :stem_language to you config.yaml with one of
these values:

  danish, dutch, english, finnish, french, german, hungarian, italian,
  norwegian, portuguese, romanian, russian, spanish, swedish, turkish

I've had this patch running on my 100K+ mailbox for two years with no
problems so far. Only tested with German, though.

IMPORTANT: after changing the language, you must re-index all of your
mail!

See http://xapian.org/docs/stemming.html for more information on how
Xapian performs stemming.