Add optional reading statistics message

This pull request adds optional reading statistics to jetzt. When reading is complete, a hidden message will be created in the DOM showing the number of words that have been read, and the duration elapsed in seconds.

Though this message is hidden by default, it can then be shown (or rather, toggled), by using the `/` or `?` key. Screenshot below:

[](https://gist.githubusercontent.com/nslater/9554254/raw/ce90c76977b65755830f131f731e62e7ea760d9d/reading-statistics.png)

Note that the message feature is generic and could be augmented to display other useful information to the user.

This feature may close issue #61.

I license this contribution under the Apache License 2.0.