Set max-width to 42em on text blocks

This makes it easier to read, as the scan back to the next line is shorter.
This becomes very aparent in todays wide screen display era. The number 42 is
picked more or less at random, so feel free to adjust that if you want :)

After:
![After](https://dl.dropboxusercontent.com/s/dg9h1hlgl336hre/Screen%20Shot%202013-07-14%20at%2010.28.04%20AM.png)

Before:
![Before](https://dl.dropboxusercontent.com/s/7snnsskjwlg1x8o/Screen%20Shot%202013-07-14%20at%2010.28.46%20AM.png)