Wrap pretty date in a html5 time tag

As a user of this library I want to render a given date in the following format so that for example a mouseover translates the pretty date into an exact date again. This information was lost by the old way this plugin rendered the date.

    <time datetime="2013-08-02 20:44:19 CEST" title="2013-08-02 20:44:19 CEST">moments ago</time>

This pull request changes the taglib to render the given time in the above described way and ensures that all existing tests are still running while a new test ensures the correct format of the enclosing tag.