TaskQuerySet should be sortable by urgency IMO

TaskQuerySet returns an iterable object. It would make sense for me if this object is sorted by urgency or at least sortable by urgency. For this to work there should be `__lt__` defined for Task. The wished behaviour can also be accomplished by similar code by extending those classes:

```python

#!/usr/bin/env python3

from tasklib import TaskWarrior
from tasklib.task import Task
from tasklib.task import TaskQuerySet

tw = TaskWarrior(data_location='~/.task')

nexttasks = tw.tasks.pending()

class TaskSortable(Task):
def __init__(self, task):
super().__init__(task.backend)
self._data = task._data
def __lt__(self,other):
return self['urgency'] > other['urgency']

class TaskQuerySetSortable(TaskQuerySet):
def __init__(self, taskqueryset):
super().__init__(taskqueryset.backend, taskqueryset.filter_obj)
def _execute(self):
tasks = []
for task in super()._execute():
tasks.append(TaskSortable(task))
return tasks

if not nexttasks:
print('No current tasks')
else:
for task in sorted(TaskQuerySetSortable(nexttasks)):
print("%s %s %s, " % (task['id'],task['description'],task['urgency']))
```