[ar] Increment line height on titles for Arabic language 

I added the line-height for all the type of title h1,h2,h3,h4,h5,h6 to fix the same font for all:

http://i.imgur.com/z70Nrv8.png
http://i.imgur.com/4sQm8M5.png
http://i.imgur.com/LPjxIAa.png
We should make this value unitless (see http://meyerweb.com/eric/thoughts/2006/02/08/unitless-line-heights/).