Duplicate price field names should be checked within set, not across all sets

Trying to create a price field with the same name as one in another price set throws the error "Name Already Exists in Database."

Duplicates should be permitted across price sets, even if not within price sets (e.g. different events may have price fields called "Sponsorship", with different sponsorship levels available for each event).

Issue discussed and partial patch provided here:
http://forum.civicrm.org/index.php/topic,6434.0.html