def get_invoices(options = {})

      request_params = {}

      request_params[:InvoiceID]      = options[:invoice_id] if options[:invoice_id]
      request_params[:InvoiceNumber]  = options[:invoice_number] if options[:invoice_number]
      request_params[:order]          = options[:order] if options[:order]
      request_params[:ModifiedAfter]  = options[:modified_since] if options[:modified_since]
      request_params[:IDs]            = Array(options[:invoice_ids]).join(",") if options[:invoice_ids]
      request_params[:InvoiceNumbers] = Array(options[:invoice_numbers]).join(",") if options[:invoice_numbers]
      request_params[:ContactIDs]     = Array(options[:contact_ids]).join(",") if options[:contact_ids]
      request_params[:page]           = options[:page]  if options[:page]

      request_params[:where]          = options[:where] if options[:where]

      response_xml = http_get(@client, "#{@xero_url}/Invoices", request_params)

      parse_response(response_xml, {:request_params => request_params}, {:request_signature => 'GET/Invoices'})
    end
def get_invoices(options = {})

      request_params = {}

      request_params[:InvoiceID]      = options[:invoice_id] if options[:invoice_id]
      request_params[:InvoiceNumber]  = options[:invoice_number] if options[:invoice_number]
      request_params[:order]          = options[:order] if options[:order]
      request_params[:ModifiedAfter]  = options[:modified_since] if options[:modified_since]
      request_params[:IDs]            = Array(options[:invoice_ids]).join(",") if options[:invoice_ids]
      request_params[:InvoiceNumbers] = Array(options[:invoice_numbers]).join(",") if options[:invoice_numbers]
      request_params[:ContactIDs]     = Array(options[:contact_ids]).join(",") if options[:contact_ids]
      request_params[:page]           = options[:page]  if options[:page]

      request_params[:where]          = options[:where] if options[:where]

      response_xml = http_get(@client, "#{@xero_url}/Invoices", request_params)

      parse_response(response_xml, {:request_params => request_params}, {:request_signature => 'GET/Invoices'})
    end
Retrieves all invoices from Xero

 Usage : get_invoices
         get_invoices(:invoice_id => "297c2dc5-cc47-4afd-8ec8-74990b8761e9")
         get_invoices(:invoice_number => "175")
         get_invoices(:contact_ids => ["297c2dc5-cc47-4afd-8ec8-74990b8761e9"] )

 Note  : modified_since is in UTC format (i.e. Brisbane is UTC+10)