Added a title element

* Added a title element available in the xml to display a title with/without the weight.
* My first idea was to make something completely independent, so the rendering would have be only in the title.py. However, the mentoring and title object are strongly related themself since the Title depends on the Mentoring.weight and that we won't use the <title> element anywhere else in the document since it won't make sense cause of the weight thing. Note that we could have also put this in a simple attribute of the <mentoring> element, but perhaps this element will change in the future and be more complex? 

In any case, let me know if you want me to change something.