Issue/252

@julen can you review this PR?
I'll add Monthly activity chart in a next commit to close issue #252.
This doesn't feel good here. You probably want to create a `task_rate_for()`/`get_task_rate()` method for users, which will take the task type as an argument:
```
task.rate = user.task_rate_for(task.task_type)
```

If you still decide not to create such a method and keep the dictionary, you should define the mapping out of the loop.