perfdata on graphs

New style for writing perfdata "20;30..50;60..100" or the old style "20;30:50;60:100" breaks the rrd graphs and gives parsing errors. Plainly writing "20;30;60" works fine for graphs and it is possible through add_perfdata method. Only caveat is if that method is used, data it not checked. I added check_perfdata_as_metric method which checks the perfdata, and exits with the proper status code.