Change to DateTimeOffset for OData 4

See discussion here:
http://scn.sap.com/thread/3441583

See examples here:
http://msdn.microsoft.com/en-us/library/ff478141.aspx

This hopefully will make it work with Tableau 8.2
Merge pull request #2 from scraperwiki/date-format

Change to DateTimeOffset for OData 4