andbug shell crashes on shlex exception

```
## AndBug (C) 2011 Scott W. Dunlop <swdunlop@gmail.com>
>> navi\
## ValueError: No escaped character
   -- File "/home/scott/andbug/lib/andbug/command.py", line 152, in perform
      act(self, *args, **kwargs)
   -- File "/home/scott/andbug/lib/andbug/cmd/shell.py", line 50, in shell
      cmd = shlex.split(input())
```
