ES6 refactor

[purrplingcat]

Refactor floating code to OOP by JS ES6. Refator following:

• Core
• PluginRegistry
• Separate logEvent() to own class and refactor
• Plugin API
  • Plugins must have function init() as mandatory
  • init() has no madatory parameters
• Remove stats from core and:
  • commandHandled counter in Commander class as read-only property
  • Reconnect counter in Core class as read-only property
• Remove deprecated methods and properties
• Separate built-in commands to own js files
• Refactor logger
  • Write derive() function for logger derivation
• Access all classes by purrplingbot.js instead of instance of core
  • Instance of core placed in modules.exports.INSTANCE. This property may not exists if no instance created.
• Move global constants to separated file constants.js