FIX: MB Trading dialect version 6.2.6

Add partial support for MB Trading FIX dialenct 6.2.6 as specified in:

  http://www.mbtrading.com/MBT%20FIX%20Gateway%20Specs%20(v6.2.6).pdf

Signed-off-by: Pekka Enberg <penberg@iki.fi>