Add support for sending unsolicited messages from a device

[FreddieAkeroyd]

This is used, for example, in the simulated gamry device. It sends a "charge complete" unsolicited message some seconds after being sent a command to "start charging".

The PR adds a unsolicited_reply() method so code like the following in the device StreamInterface can accomplish this:

# function to send unsolicited message when charging complete
def charged(self):
    self.handler.unsolicited_reply("STOPPED")

# Device commanded to start charging
def start_charging(self):
    t = Timer(self.charging_time, self.charged)
    t.start()
    return "STARTED"

[DominicOram]

@mattclarke, would you like to take a look at this? Otherwise I'm happy to merge it.