Provide NullFormatter.
Document processing logic of formatter and reporter.
Issue #27: Row should support .get() to be more dict-like
  * model.Row: Changed Ctor parameter ordering, move seldom used to the end.
  * model.Row: Add methods .get(), .as_dict() and len operator (related to: #27).
  * Introduce ``behave.compat`` as compatibility layer for Python versions.