Big refactor, removed callbacks and updated to ES6 classes

• Removed all callbacks, only Promise is supported now.
• Removed authorise method in favor of authorize.
• Removed callback method, verify will now throw an Error.
• Removed the logger, use DEBUG=node-bigcommerce:* as an environment variable to get debug messages.
• Dropped support of node-v4 for missing class implementation.
• Removed catalogue hack, please use apiVersion configuration property.