added missing timedelta import

```
$ python2 ./gcalcli --detail-descr agenda
Traceback (most recent call last):
  File "./gcalcli", line 1899, in <module>
    BowChickaWowWow()
  File "./gcalcli", line 1779, in BowChickaWowWow
    gcal.AgendaQuery()
  File "./gcalcli", line 1066, in AgendaQuery
    end = (start + timedelta(days=self.agendaLength))
NameError: global name 'timedelta' is not defined
```

Just missing timedelta import.