Added Twitter Chat Sessions

Import Jobs are:
- rake chatsessions:add (weekly or daily)
- rake questions:import (ev. 15 minutes)
- rake answers:import (ev. 15 minutes)

Path:
/chatsessions/:id

Actual there is still a problem with li:before styling
Let's change ```DateTime.now.cweek``` to 
```
def chat_number
((DateTime.now - "February 5, 2014".to_datetime).to_i)/7
end
```

Since the first chat was on February 5. Does the make sense?