v0.5.0 - Better voice, bot accounts, more REST

• New constructor Discord::from_bot_token for bot accounts
• Errors decoding optional fields are no longer silently ignored
• The ID types now wrap u64s instead of Strings
• Voice overhaul
  • Stereo support
  • Voice receive support
  • Encrypted voice support
  • Multi-server voice support
  • State is retained even if connection is lost
  • New examples: dj and voice_receive
• Rest API additions
  • get_voice_regions, move_member_voice
  • get_servers, get_bans, add_ban, remove_ban
  • send_file
  • edit_member_roles, kick_member
  • edit_server, edit_profile (use read_image for avatars)
  • create_invite, get_server_invites, get_channel_invites
  • create_server now has an icon parameter
• Various model updates for new and changed Discord functionality