Added a few features I needed

Hi, I started playing with yowsup today and there were few features I missed, so I implemented them:
  * A more descriptive disconnection reason in case of a stream error
  * Signals for participants joining / leaving groups
 
As I stated before I only started playing with yowsup today and I don't have deep understanding of the library. I tried not to break the design as much as possible. However, if I did break the design, coding conventions, etc.. please give me feedback and I will fix it.