Added the DLE EOT querying command.

Added a function to check whether the printer is online or not, as well
as a reading method for USB printers.

src/escpos/printer.py

@@ -84,6 +84,10 @@ class Usb(Escpos):
         """
         self.device.write(self.out_ep, msg, self.timeout)
 
+    def _read(self):
+        """ Reads a data buffer and returns it to the caller. """
+        return self.device.read(self.in_ep, 16)
+
     def close(self):
         """ Release USB interface """
         if self.device: