Update CheckoutStep_Address.php: function setbillingaddress

Remove $step as it is not defined in the setbillingaddress function.  
This change seems to work well in browser, however, testBillingAddress in SteppedCheckoutTest fails.
Merge pull request #284 from AntonyThorpe/patch-1

Update CheckoutStep_Address.php: function setbillingaddress
Update _config.php

Additional unnecessary argument.  The function setupSteps, within class SteppedCheckout, only requires $steps.
Merge pull request #285 from AntonyThorpe/patch-2

Update _config.php
Added a bunch of add-on modules to README
Try again
Update README.md
README update
Reordered the README file a bit more
Update README.md
Update README.md
Update CONTRIBUTING.md
Documentation overhaul. Updated to be structured better, and more consistent with the current state of the module.
Updated payment documentation
Further documentation improvements
Updated CartForm to work with Buyables, rather than just Product.
Updated Custom Products documentation to include some additional notes about what to implement.
Thanks @wildflower for finding this.
Update Custom_Checkout.md for typo

Typo
Update Multi_Step_Checkout.md for Additional Form Fields

Demonstrate through a simple example how to add an additional form field to a Multi-Step Checkout.
Update CheckoutStep_Address.php

Use different extension names for the ShippingAddressForm and the BillingAddressForm
Update ProductCategory.ss to include Sorter

Include Sorter.ss from burnbright/silverstripe-listsorter.  Note: SortLinks doesn't exist in SS Shop.