CRM-13630 fixes and Premium improvement

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* CRM-13630: Minimum contribution amount check for Premiums fails when online contribution form uses a Membership price set.
  http://issues.civicrm.org/jira/browse/CRM-13630
Related fix for CRM-13630

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* CRM-13630: Minimum contribution amount check for Premiums fails when online contribution form uses a Membership price set.
  http://issues.civicrm.org/jira/browse/CRM-13630