changelog: use time elements for release dates

This changes the visual presentation ever so slightly:

![Before](https://f.cloud.github.com/assets/210406/2197655/a8a77f8a-98be-11e3-889d-1e93667cd4ba.png "Before")

![After](https://f.cloud.github.com/assets/210406/2197656/adccb142-98be-11e3-9117-f172634e3d62.png "After")

I was able to avoid the need for the `document.createElement('time')` trick (for old IEs) by having the time element inherit all its styles from its parent element. This involved reducing the font-size of the parent element from 12px to 11px. To offset this change, I bumped up the en dash to an em dash. The result is satisfactory visually, and the markup is significantly more meaningful as a result.

It's safe to change the styling of `.timestamp` elements as they're used exclusively for marking up release dates in the change log.