Date utc changes

A few bits:

  * Remove times from date subrecords.  Now only support granularity down to the day.

  * Add a new 'timestamp' field to event records.  You can use this instead of linking to a date subrecord if you want to capture a particular moment in time.

  * Allow date subrecords to support BC dates by prefixing them with a '-'

  * Modifications to the EAC exporter to support the changed date subrecords.  Added a unit test for the functionality I affected.