improves deprecation warning message and allows passing default values

* states the key and model for which the human_attribute_name is
 inquired
* have the raise and default option only as a fallback when nothing else
 is supplied. Doing this allows for model classes to pass additional
 attributes.
Merge pull request #87 from opf/feature/rails3_human_attribute_name_improvement

improves deprecation warning message and allows passing default values