Fix line height for all headings

[Before](http://i.imgur.com/MOuj3SX.png), [After](http://i.imgur.com/6TFAupw.png). According to [this resource](http://practicaltypography.com/line-spacing.html) it should be between 120% and 145%, I went with 120% and just the headings for now for minimal intrusion. Feel free to increase it.