Implement sing module for bobbit-0.2.x

@camicarballo Created a **sing** module for `bobbit-0.1.x`: https://github.com/pbui/bobbit/blob/bobbit-0.1.x/modules/sing.py

Unfortunately it did not work due to the web service blocking the VPS for bobbit.  Moreover, it was not ported to `bobbit-0.2.x`.  I would like for someone to implement this functionality:

```
<- !sing [song title]
-> Random lyric from song
```