problem with Model.select(SQL('*'))...
Peewee does not currently support this, unfortunately. You can always pass in the model class to `.select()` and it will select the appropriate columns.

Hi,

I have a very complex Query and want to use SQL('*') to select all fields to avoid specifying all fields and making the query even longer.
Very basic example:

```
from peewee import SqliteDatabase, PrimaryKeyField, CharField, Model, SQL, fn

db = SqliteDatabase(':memory:')

class Model1(Model):
    id = PrimaryKeyField()
    column = CharField()

    class Meta(object):
    database = db

Model1.create_table()
Model1.create(id=1, column='bar')
Model1.create(id=2, column='foo')
Model1.create(id=3, column='baz')

res = Model1.select(SQL("*"))
print res
for a in res:
    print a
```

which results in the following error:

```
<class '__main__.Model1'> SELECT * FROM "model1" AS t1 []
  File "<path>/tests/bla.py", line 30, in <module>
    for a in res:
  File "<path>/env_local/local/lib/python2.7/site-packages/peewee.py", line 1615, in next
    obj = self.iterate()
  File "<path>/env_local/local/lib/python2.7/site-packages/peewee.py", line 1601, in iterate
    self.initialize(self.cursor.description)
  File "<path>/env_local/local/lib/python2.7/site-packages/peewee.py", line 1643, in initialize
    select_column = self.column_meta[i]
IndexError: list index out of range
```

i'm using the current master.

Thanks
Goir
