Provide ability to post playlogs from Engine Core, plus setting relevant metadata via Engine

See meta#42

• Build payload with new JSON builder
• Post playlog via new http.post method
• Ability to remotly update certain configuration (e.g. info on fallback show)
• Issue playlog for fallback sources
• Issue playlog for annotated queue items
• Issue playlog for input streams, metadata provided by Engine using a Liquidsoap server function
• Issue playlog for audio inputs, metadata provided by Engine using a Liquidsoap server function
• New Liquidsoap 2.1 metadata event handlers
  • Apply for streams
  • Apply for audio inputs
• Merge and insert metadata
  • Basic functions and implementation for general streams
  • Special case: Stream is continuously updating metadata remotely, apply advanced merge-strategy See Metadata Handling: Some streams continuously up... (#26)
• Remove back-channel config
• Update docs
• Test cases where applicable
• Testing