v0.4.0 - Better voice, error handling

• Simplified voice module API and added youtube-dl support.
• Added Discord::new_cache which will cache and reuse past login tokens. Use is recommended.
• Improved error handling, with better messages and more descriptive entries in the Error enum:
  • Error::Status now also contains the json response describing the error.
  • Error::RateLimited added, including how long to wait before retrying.
  • Error::Protocol added for protocol misunderstandings.
• Updated for protocol changes:
  • Removed mentions array from send_message and edit_message.
  • Removed UserSettings::muted_channels, added instead UserServerSettings.
  • Added PublicChannel::bitrate field.
  • Added LiveServer::verification_level and member_count, Server::verification_level, and removed Server::joined_at.
• Added REST endpoints:
  • create/leave/delete server
  • get/accept/delete invite
  • edit channel