Add a duration remaining message toggle

This pull request adds the ability to toggle a message displaying the approximate time remaining in seconds till reading is completed.

This message is shown using the same DOM element and interface command (the / or ? key) as introduced in pull request #62.

This is how this feature looks:

![duration-remaining](https://f.cloud.github.com/assets/23469/2431177/fc6ae966-ad1f-11e3-9cbe-f7af9753b62b.png)

Possible feature upgrades include showing minutes and seconds if the number of seconds exceeds 60s, and automatic updating of the remaining time per second for as long as the message display is open.

This pull request is intended to close issue #25.