message: reduce amount of logging that happens by default
  task begin/ends no longer log, now debug
diff: @@ -1050,7 +1050,7 @@ function Botkit(configuration) {
  };
  this.taskEnded = function() {
  - botkit.log('[End] ', this.id, ' Task for ',
  + botkit.debug('[End] ', this.id, ' Task for ',
  this.source_message.user, 'in', this.source_message.channel);
  this.status = 'completed';
  @@ -1447,7 +1447,7 @@ function Botkit(configuration) {
  var task = new Task(bot, message, this);
  task.id = botkit.taskCount++;
  - botkit.log('[Start] ', task.id, ' Task for ', message.user, 'in', message.channel);
  + botkit.debug('[Start] ', task.id, ' Task for ', message.user, 'in', message.channel);
  var convo = task.startConversation(message);
  