CRM-12923 - crm.backbone.js - Add isModified (in addition to isSaved)

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* CRM-12923: Create HTML prototype of "Job Position" UI
  http://issues.civicrm.org/jira/browse/CRM-12923
Merge pull request #1265 from totten/master-bb-ismodified

CRM-12923 - crm.backbone.js - Add isModified (in addition to isSaved)