Feature:

  * Add notes about events in `api` document
  * Update copyright year
  * Cleanup docs

References:

  * Feature request: https://github.com/yagop/node-telegram-bot-api/issues/289
  * PR: https://github.com/yagop/node-telegram-bot-api/pull/291
  * Requested-by: @preco21 
  * PR-by: @preco21