Add multi item push to fix issue #39

[jordanwalsh23]

Add support for multi item push using the following code:

       var sampleItem = {
            Code: 'Item-' + Math.random(),
            Name: 'Fully Tracked Item',
            Description: '2014 Merino Sweater',
            PurchaseDescription: '2014 Merino Sweater',
            PurchaseDetails: {
                UnitPrice: 149.00,
                AccountCode: salesAccount
            },
            SalesDetails: {
                UnitPrice: 299.00,
                AccountCode: salesAccount
            }
        };            
            
            var items = [];

            for (var i = 0; i < 10; i++) {
                sampleItem.Code = 'Item-' + Math.random();
                items.push(currentApp.core.items.newItem(sampleItem));
            }

            currentApp.core.items.saveItems(items)
                .then(function(response) {
                    expect(response.entities).to.have.length.greaterThan(9);
                    done();
                })
                .catch(function(err) {
                    done(wrapError(err));
                })