ExtQueryWrapper raises exception when selecting fn.NOW()
Nice catch, you're right.

When using something like this: `Record.select(Record.datetime, fn.NOW()).tuples()`, on iteration exception gets thrown:

```
File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 1686, in next
    obj = self.iterate()
File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 1672, in iterate
    self.initialize(self.cursor.description)
File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 1724, in initialize
    isinstance(select_column.arguments[0], Field)):
IndexError: tuple index out of range
```

The place in current master is: https://github.com/coleifer/peewee/blob/master/peewee.py#L1729

Should be checked that `len(select_column.arguments) > 0`
