Add Dionysis Grigoropoulos to the authors list

AUTHORS

-MAIN DEVELOPERS:
-Nikos Skalkotos <skalkoto@grnet.gr>
-
-CONTRIBUTORS:
-Alex Pyrgiotis <apyrgio@cslab.ece.ntua.gr>
-Vangelis Koukis <vkoukis@grnet.gr>
-Giannis Spiliopoulos <gspilio@admin.grnet.gr>
-Konstantinos Tompoulidis <kostikas@grnet.gr>
+Nikos Skalkotos
+Alex Pyrgiotis
+Vangelis Koukis
+Giannis Spiliopoulos
+Konstantinos Tompoulidis
+Dionysis Grigoropoulos