CRM-13880 Make payment instrument field required in all forms. Set to Check as default. API sets payment instrument to check if none is provided.

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* CRM-13880: Make payment instrument required for all contribution, event registration and membership forms
  http://issues.civicrm.org/jira/browse/CRM-13880
CRM-13880 - Update payment instrument help text
Merge pull request #2126 from davecivicrm/CRM-13880

CRM-13880 Make payment instrument field required in all forms. Set to Ch...