using plural forms in a place where duganov reported, all devs, patchers have a look; also updating pot/po to match plural forms usage and some new strings changes from greek po [as I go on]; we now also have make gajim.pot to produce the pot with the help of intltool

I updated partially greek gajim.po and fixed some strings so I commit po and pot. there is a lot of job to be done before all strings are reviewed and become better

pot and po were updated in case translators wanna hack they can get the latest po by browsing to the file they want [po]