[tell] Fixes #418. Error when not enough args given

```
<Flyte> .tell
<Lurk> AttributeError: 'NoneType' object has no attribute 'split' (file "/home/flyte/workspace/willie/ve/local/lib/python2.7/site-packages/willie/modules/tell.py", line 93, in f_remind)
<Flyte> .tell Flyte
<Lurk> ValueError: need more than 1 value to unpack (file "/home/flyte/workspace/willie/ve/local/lib/python2.7/site-packages/willie/modules/tell.py", line 93, in f_remind)
```