Introduce arakoon_user_function

This exposes Arakoon's user function interface.

Signed-off-by: Arne Redlich <arne.redlich@googlemail.com>
Merge branch '1.2' of https://github.com/aredlich/crakoon into PR15

* '1.2' of https://github.com/aredlich/crakoon:
  Introduce arakoon_user_function