CRM-13120 - Reproduce 'null' problem with a core API

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* CRM-13120:
  http://issues.civicrm.org/jira/browse/CRM-13120
CRM-13120 - APIv3 - Don't return "null" from "create" action

 * Split CRM_Core_HTMLInputCoder in three:
   * CRM_Utils_API_AbstractFieldCoder
   * CRM_Utils_API_HTMLInputCoder
   * CRM_Core_HTMLInputCoder (for backward compatibility)
 * Add CRM_Utils_API_NullOutputCoder (based on AbstractFieldCoder)

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* CRM-13120:
  http://issues.civicrm.org/jira/browse/CRM-13120
Merge pull request #1275 from eileenmcnaughton/CRM-12108

CRM-12108 minor improvement - simplify ipn.php by putting logic in the
Merge pull request #1307 from eileenmcnaughton/CRM-12108

 CRM-12108 fix bug introduced when rebasing PR
Merge pull request #1308 from totten/master-apicreate-nulls

CRM-13120 - APIv3 - Don't return "null" from "create" action
CRM-13067 discussion bao / api restructure
CRM-13109 move getContactID to CRM_Core_Form so it's available to other forms
CRM-13067 tweak compulsory fields
CRM-13067 update
CRM-13067 allow cid in URL for membership payments
CRM-13067 fix required params on membership payment
CRM-13067 tweak on membership
CRM-13067 further fixes