Refactoring of the MacroStep class. Fixed missing require.

* Class MacroStep: method validate_params is now private, the signature
of expand method has changed.
* macro-step.rb: added a missing require
'exceptions'.
* macro-step_spec.rb: updated to taken into account the
above change. Added test case when a macro is called with unknown
argument.
* Class MacroCollection#render_steps updated for the above
change.