Improve the efficiency of the get_shipments call

After consulting with the ShipStation developers they informed us the CreateDate for Shipment could be used as a proxy for ShipDate which does not contain time information.  Now we can return just the shipments that have shipped since we last checked which also allows us to comply with the new API limits set by ShipStation.

[Fixes IN-306]
Do not use UTC for timestamps

ShipStation incorrectly treats all times as PST so UTC time will often result in the wrong day (if it's after midnight UTC)

[Fixes IN-308]