fix when not using named cursor
Interesting, what version of psycopg2 are you using?
 missed my environment
ubuntu 14.04
python 2.7.6
psycopg2 is 2.4.5 from python-psycopg2 package

It could be this situation.

```
import psycopg2

name = None
connection = psycopg2.connect('dbname=test user=test')
cursor = connection.cursor(name=name)
```

```
Traceback (most recent call last):
  File "test.py", line 5, in <module>
    cursor = connection.cursor(name=name)
TypeError: argument 1 must be string, not None
```
 Ew, just a tip but you should definitely be using pip and not the Ubuntu psycpog2 package.

That said, seems like a fine PR so I'll go ahead and merge.

I'm using peewee on my projects. Thanks. :)

I wrote a simple connection to the postgresql server without named cursor.

```
from peewee import *  # NOQA
from playhouse.postgres_ext import *  # NOQA

database = PostgresqlExtDatabase('test', user='test')


class MyModel(Model):
    name = CharField()
    value = CharField()

    class Meta:
        database = database


MyModel.create_table()
```

An error occurred.

```
Traceback (most recent call last):
  File "test.py", line 15, in <module>
    MyModel.create_table()
  File "/home/vagrant/src/peewee/peewee.py", line 4010, in create_table
    db.create_table(cls)
  File "/home/vagrant/src/peewee/peewee.py", line 3115, in create_table
    return self.execute_sql(*qc.create_table(model_class, safe))
  File "/home/vagrant/src/peewee/playhouse/postgres_ext.py", line 356, in execute_sql
    cursor = self.get_cursor()
  File "/home/vagrant/src/peewee/playhouse/postgres_ext.py", line 343, in get_cursor
    return self.get_conn().cursor(name=name)
TypeError: argument 1 must be string, not None
```
