Consider burden of each issue

We could estimate each issue using its title, just like [Scrum for Trello](http://scrumfortrello.com/). This would generate an accurater burndown graph.

Like:

```
Add auto-refresh to page after min of inactivity
```

estimated as 3 points...

```
(3) Add auto-refresh to page after min of inactivity
```

With this change, users should be able to set (or define a system wide one) the default estimation for issues without `(x) ` prefix.