added linearized gamma workflow for better/brighter results and fixed some issues

+ fixed an e-notice with IPTC: added property $validIptcTags and changed code in iptcPrepareData()
+ fixed / changed bgcolor in imRotate()
+ changed default in imSharpen from medium to soft to be compatible with default settings in site/config.php
+ added linearized workflow with gamma correction for gif and jpeg, but not for png, because it doesn't support alpha transparency. (more on this can be read/seen here: http://www.4p8.com/eric.brasseur/gamma.html)