started changing skolemization to create correct fol formulas, at the moment the toString method fails (and other algorithms will therefore also fail). added detection of strong quantifiers instead of skolemizing the endsequent to work around issue 220

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1394 85d21ef8-add0-11de-ae77-5bfb9c5a3f34