Add Use External ID config option
Validate if SSO needs external_id param based on config option
Added Helper function to retrieve use_external_id value from config
Replaced how external_id value is retrieved. Now calls to the new helper func
Merge pull request #28 from zendesk/fix-externalid

Enable/disable external_id param while usign SSO
Merge branch 'master' into daniel/respond_with_403_if_no_headers
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