Documentation, Shipment methods

Able to create a full shipping label. Documentation added to all methods. Able to return either Array or Object from most requests. Some methods require an object as the input.

$shipment = $this->EasyPost->ShipmentCreate(array(
		    	'to_address' => $this->EasyPost->AddressCreate($to),
		    	'from_address' => $this->EasyPost->AddressCreate($from),
		    	'parcel' => $this->EasyPost->ParcelCreate($parcel)
	    	));
$this->EasyPost->ShipmentBuy($shipment,'lowest');
echo $this->EasyPost->PostageLabelURL($shipment);

Signed-off-by: Derek Smart <dereksmart@earthlink.net>