Make command replies consistent with requests

[macvenez]

resolves #22

I added $ character to each command reply.
It might be unnecessary in some conditions like replies to commands with wrong number of arguments.
In general I think it's good anyway to easily distiguish between messages

[antirez]

Hello @macvenez, checking your PR I think we could go the Redis protocol route. What about:

Starting the reply with "+" in case of successful reply. "-" in case of errors?
Also, please note that there is a small problem in the PR in the case of the .join() call when generating the command list.

[macvenez]

Yes! That's an even better idea, I'll edit it accordingly.

Can you illustrate what's going to happen with .join?

That's an example code:

LoRaPresets = ['preset1', 'preset2', 'preset3']
result = "-Valid presets: " + ", ".join(x for x in LoRaPresets)
print(result)

Shouldn't the output be:
-Valid presets: preset1, preset2, preset3 ? I think that's a correct output, right? Am I missing something (maybe in micropython something different occurs)?
Thanks!

[antirez]

Great. Thanks. The problem with join is only here AFAIK:

    send_reply("$, ".join(self.fw.keychain.list_keys()))

>>> "$, ".join(["a","b","c"])
'a$, b$, c'

[macvenez]

Alright, now I see it, that should be easily fixable with:
send_reply("$"+", ".join(self.fw.keychain.list_keys()))

of course that $ should be a +