Allow contact's birthdate in views to be customisable, sortable, filterable, etc.

A contact's birthdate is currently processed by View's as just a regular text field. Since it does not recognise that it is a date, the birthdate isn't able to be manipulated in the usual way that date/time fields are in Views, eg. to be rewritten to a different format, to be sorted or filtered by date.

The attached patch sets handlers for the field birth_date to the civicrm set of datetime handlers.