New Feature: Implemented clickvote

[Canuma]

As requested by Radio Browser, I implemented a clickvote Feature. It is disabled by default, enable it in server.py. I am however not quite sure if this actually works as intended.

[Essoloani]

Hello,
Do you plan to offer your test version soon?

[Canuma]

Hi,
I don't have plans to release my own versions. If you are familiar with git, you can checkout the clickvote branch from my repo.
Be aware that this is based on @milaq master, thus it will not work on older Yamaha AVRs (and probably others) due to StationIDs being to long. I have another branch fixing this problem too.