Added possibility to learn ham
Make configurable and autocreate folder

Ham learning is now more configurable and added more parameter to the
ham section:
* create     autocreate the ham folder if not exists
* marker     marker which should be removed from ham
* folder     the ham folder (wihtin inbox)
* limit      limit to the first x elments in the folder if > 0
Merge pull request #16 from m42e/learnham

Added possibility to learn ham