Code improvements and do not add "- Any -" if required field.

Hello guilopes,

after the last great improvement I had some further problems using the module for other entity types than "node". Furthermore the general problem existed that the "- Any -" selection appeared even if the field was set required, which is agains Drupal standards for radio buttons and hard to understand for users logically.

I fixed the problems (selected target id was not saved correctly) and added a check to add the "- Any -" option only if the field is optional.

Some explanations for you:

• The fixes at "#delta" were required to make it work correctly.
• We're now using "array_pop" instead of [0] because the index of the selected value may be different
• $delta is being counted dynamically now
• entityreference_rendered_widget_attach_fields() needed a further parameter "instance" to detect if the field instace is marked required
• $entity_type was renamed to $target_entity_type to make it more clear that not the parent entity is meant with this variable but the target entity (that was a bit confusing for me)

I've tested the result on several pages and it works great. Thank you for your great work at this module