hstore extension is required to initialise a postgresql database
I'm not really sure what to do here -- it's kind of on the database user to install the extension if they want to use it.
 The point is I don't want to use it, it's forcing me to install it to use peewee.
 Oh, I see.
 I've added a `register_hstore` parameter to `PostgresqlExtDatabase`. The default is `True` to avoid breaking backwards compatibility.

I've been meaning to write this one up for a while.. I've just installed the extension every time to avoid this one :)

```
Traceback (most recent call last):
  File "peewee_bug.py", line 22, in <module>
    MyModel.create_table()
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 3423, in create_table
    db.create_table(cls)
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2788, in create_table
    return self.execute_sql(*qc.create_table(model_class, safe))
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 288, in execute_sql
    self.commit()
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2600, in __exit__
    reraise(new_type, new_type(*exc_value.args), traceback)
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 279, in execute_sql
    cursor = self.get_cursor()
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 266, in get_cursor
    return self.get_conn().cursor(name=name)
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2680, in get_conn
    self.connect()
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2667, in connect
    self.__local.closed = False
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2600, in __exit__
    reraise(new_type, new_type(*exc_value.args), traceback)
  File "/Users/alexlatchford/Code/peewee/peewee.py", line 2666, in connect
    **self.connect_kwargs)
  File "/Users/alexlatchford/Code/peewee/playhouse/postgres_ext.py", line 293, in _connect
    register_hstore(conn, globally=True)
  File "/Library/Python/2.7/site-packages/psycopg2/extras.py", line 775, in register_hstore
    "hstore type not found in the database. "
peewee.ProgrammingError: hstore type not found in the database. please install it from your 'contrib/hstore.sql' file
```
