OPC css cleanup and general improvements.

Local.xml: Added opc.js through phtml (before body end) to be able to override opcheckout.js
One Page Checkout cleanup and generales css improvements.
remember_me_tooltip.phtml usability fix (used in opc too).
Unified tooltips experience also in OPC.
Added product image and product sku to One Page Checkout Review Step.
Checkout Success/Failure css definitions.
Defined cart-empty section on Cart page.