CRM-13492 - update buildAmount hook code to match price set changes

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* CRM-13492: buildAmount Hook functionality changed -- difficult to use / undocumented
  http://issues.civicrm.org/jira/browse/CRM-13492
Merge pull request #65 from dlobo/CRM-13492

CRM-13492 - update buildAmount hook code to match price set changes