Issue #1349 - Add createdAt function to ObjectIdField
Issue #1411 - dirty_? flag in Record and Meta as in Mapper
Replaced all uses of MongoId with ObjectIdPk in test code.
Issue #1348 - Add lifecycle callbacks to MongoMetaRecord.update
Minor cleanup re PR feedback
Merge pull request #1464 from lift/tcn_issue_1349

Issues 1349, 1411, and 1348