Show last updated in words on front page.

Front page now shows the 'last edited at' field in an easier (for humans)
to grasp format, namely in words as opposed to a simple timestamp.
Timestamp is also added to the article show page.

This should make it easier to get an impression of how actively contributions actually happen.

Moment.js is used to calculate the time ago in words.

![image](https://cloud.githubusercontent.com/assets/1413365/2937034/542aa636-d882-11e3-8c9a-0117d093c1ee.png)