Add support for tstamp-fields
The current version of Doctrine comes with a template "Doctrine_Template_Timestampable".
We can probably use this to support TYPO3s tstamp-columns (crdate, tstamp)
tx_doctrine_TcaImport should add appropriate act-as definition to models with timestamp-fields.
Note: For the fields starttime/endtime some custom search method could be added to the model classes. But that's probably not as critical for now.