LookupError: unknown encoding: utf8mb4
This error is coming from the MySQL driver so I suspect that peewee isn't the issue... But.

Does peewee need to run any special queries before the create table query?

I'm not sure whether the problem is linked to my specific settings, but it seems I can't do `table_exists` on connection with a mysql database using `charset = utf8mb4`.

I previously altered my database with 

```
SET NAMES utf8mb4;
ALTER DATABASE my_database CHARACTER SET = utf8mb4 COLLATE = utf8mb4_unicode_ci;
```

but 

```
  File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 4548, in table_exists
    return cls._meta.db_table in cls._meta.database.get_tables(**kwargs)
  File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 3879, in get_tables
    return [row for row, in self.execute_sql('SHOW TABLES')]
  File "/usr/local/lib/python2.7/dist-packages/peewee.py", line 3454, in execute_sql
    cursor.execute(sql, params or ())
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
LookupError: unknown encoding: utf8mb4
```
