Thunderbird 66+ Compatibility

• Adds Thunderbird 66 Compatibility:
  • Converts TreeBoxObject into XULTreeElement (#162)
  • Adopts StreamListener API Changes (#163)
  • Adopts ProtocolHandler API Changes (#164)
• Fixes unloading the addon (#166 and #165)
• Makes parser to handle Identifiers as case sensitive.
• Fixes SHA iterations (#161)
• Activates Hungarian Locale
• Removes Obsolete Files
• Updates Credits and include Contributors.md