<code>/<tt> elements should wrap since they're used inline

This pull request changes the `code, tt` rule `white-space: pre` to `pre-wrap`. [Definitions](http://www.w3.org/TR/CSS21/text.html#white-space-prop):

> **pre**
> This value prevents user agents from collapsing sequences of white space. Lines are only broken at preserved newline characters.
> **pre-wrap**
> This value prevents user agents from collapsing sequences of white space. Lines are broken at preserved newline characters, and as necessary to fill line boxes.

Before:
![white-space: pre](https://f.cloud.github.com/assets/118362/1408033/51d61f12-3d7a-11e3-887b-5ca22839b638.png)

After:
![white-space: pre-wrap](https://f.cloud.github.com/assets/118362/1408034/54e5228e-3d7a-11e3-88e4-b92611842759.png)

See also TryGhost/Ghost#1279.