def list_clusters(options={})
      params = {
        :operation => 'ListClusters'
      }
      params.merge!(:cluster_states => options[:states]) if options[:states]
      params.merge!(:created_before => options[:created_before].to_i) if options[:created_before]
      params.merge!(:created_after => options[:created_after].to_i) if options[:created_after]
      params.merge!(:marker => options[:marker]) if options[:marker]
      aws_result = @aws_request.submit(params)
      yield aws_result if block_given?
      JSON.parse(aws_result)
    end
def list_clusters(options={})
      params = {
        :operation => 'ListClusters'
      }
      params.merge!(:cluster_states => options[:states]) if options[:states]
      params.merge!(:created_before => options[:created_before].to_i) if options[:created_before]
      params.merge!(:created_after => options[:created_after].to_i) if options[:created_after]
      params.merge!(:marker => options[:marker]) if options[:marker]
      aws_result = @aws_request.submit(params)
      yield aws_result if block_given?
      JSON.parse(aws_result)
    end
List the clusters given specified filtering

   emr.list_clusters({
     :states => ['status1', 'status2', ...],
     :created_before => Time,                # Amazon times are in UTC
     :created_after => Time,                 # Amazon times are in UTC
     :marker => 'marker'                     # Retrieve from a prior call to list_clusters
   })

 http://docs.aws.amazon.com/ElasticMapReduce/latest/API/API_ListClusters.html