add 'keyboardDidHide' notification handling

:: JSQMessagesKeyboardControllerDelegate protocol adds options `keyboardControllerDidReceiveKeyboardHideNotification:` method
:: this method is called within the completion block of the typical keyboard frame change notification handler, ensuring that the 'hide' message is received last
:: `JSQMessagesViewController` uses this delegate callback to call `jsq_setToolbarBottomLayoutGuideConstant:` with a value of `0.0f` (ensuring that a 'hidden' keyboard results in a 0-position input toolbar)
Merge pull request #646 from evilstudios/keyboard-notification-fix

Fix #645: add 'keyboardDidHide' notification handling