Pelagios Time Metadata

[ekansa]

Once @rsimon checks and OKs the basics of Open Context's Pelagios annotations, we should add time metadata to the annotated items.

It will be more immediately feasible to do this with calendar date-ranges rather than PeriodO (http://perio.do/) URIs. A the recent PeriodO workshop we discussed the need for more general regional period concepts, because we sometimes do not want to commit to a specific authority. I will take some time for PeriodO to work through how to support this need, and it will take time on our end to properly reference PeriodO and add all those URIs