Implemented custom hashref keys in the iterators for dropdowns.

Added two options to the specification:

 - iterator_value_key
 - iterator_name_key

The option overwrite the hardcoded "value" and "label" (which are kept
as default).

Implemented the lookup of hash keys in the iterators for the dropdown

Doc + changed names
Merge pull request #19 from melmothx/topic/iterators_values

Implemented custom hashref keys in the iterators for dropdowns.