Feature/check for null data

    tim@domU-12-31-39-09-90-72:~/check-graph$ ./check_graph --help
    CLI Check for Graphite

    Usage: check_graph [-n|--check-null] URL TARGET OPERATOR VALUE MINUTES
      Checks Graphite for out-of-bounds data

    Available options:
      -h,--help                Show this help text
      -n,--check-null          Check for null data for TARGET
      URL                      Graphite API URL
      TARGET                   Graphite Target Metric (pattern or name)
      OPERATOR                 '<', '<=', '==', '>' or '>='
      VALUE                    Combined with OPERATOR to check TARGET's values
      MINUTES                  Number of previous minutes (from now) to check

    tim@domU-12-31-39-09-90-72:~/check-graph$ ./check_graph 'http://metrics.staging.knewton.net:8080' 'Platform2.QA.KPIP-Processor.*.jvm.memory.heap.percentUsed' '<=' 0.95 5
    OK: Graphite values are within bounds

    tim@domU-12-31-39-09-90-72:~/check-graph$ ./check_graph -n 'http://metrics.staging.knewton.net:8080' 'Platform2.QA.KPIP-Processor.*.jvm.memory.heap.percentUsed' '<=' 0.95 5
    CRITICAL: Graphite values contain nulls Platform2.QA.KPIP-Processor.*.jvm.memory.heap.percentUsed