Arabic translation and  support for right to left languages

When any right to left language is selected (just arabic now) a "rtl" id is added to body and corresponding css styles are applied . If the choosen language is left to right the id will be removed .
I also  added to the pull request the arabic translation and an updated french one .

Special thanks to all the people that helped to translate prism-break to arabic on this pad http://pad.tn/p/prism-break-arabic :-)

ps: you can see a preview of the result here : http://tux.duckdns.org/prism-break/