Default for register_hstore in postgresql extensions should be False
Default usage of the pewee playhouse postgresql extension fails to work with an out-of-the-box postgresql database. The postgresql database ships with the hstore extension not installed, but the playhouse postgresql extension has this default setting.  
    kwargs.pop('register_hstore', True)

This causes the DB to fail to open if the user has not explicitly installed the hstore extension in their database.  This is confusing if the user does not intend to use hstore. 

At very least, this deserves a warning box in the documents to alert the user to open the PostgresqlExtDatabase with register_hstore=False unless the hstore extension is already installed in the database that they intend to use. 
