APIs added & updated:
* Retrieve invoices for a customer. See https://apidocs.chargebee.com/docs/api/invoices?lang=ruby#list_invoices_for_a_customer.
* Retrieve transactions for a customer. See https://apidocs.chargebee.com/docs/api/transactions?lang=ruby#list_transactions_for_a_customer.
* Now, a customer(without subscription) can be charged(Create invoice for Charge) for one time charges. See https://apidocs.chargebee.com/docs/api/invoices?lang=ruby#create_invoice_for_charge.
* Now, a customer(without subscription) can be charged for one time addons(Create invoice for Addon). See https://apidocs.chargebee.com/docs/api/invoices?lang=ruby#create_invoice_for_addon.
Change log & version(1.4.2) update.