fix:playhouse connect database_url error
fix:playhouse connect database_url, the max_connections is mistaken for the type of 'str', so the error occur at https://github.com/coleifer/peewee/blob/master/playhouse/pool.py#L126

example:

```
BACKEND_MYSQL="mysql+pool://root:root@127.0.0.1/torweb?max_connections=20&stale_timeout=300"
```

`max_connections` and `stale_timeout` are mistaken for the type of 'str', and error occur

```
Traceback (most recent call last):
  File "/Users/jmpews/virtualenv/torweb/lib/python3.5/site-packages/tornado/web.py", line 1422, in _execute
    result = self.prepare()
  File "/Users/jmpews/Desktop/codesnippet/python/torweb/handlers/basehandlers/basehandler.py", line 18, in prepare
    db_mysql.connect()
  File "/Users/jmpews/virtualenv/torweb/lib/python3.5/site-packages/peewee.py", line 3402, in connect
    **self.connect_kwargs)
  File "/Users/jmpews/virtualenv/torweb/lib/python3.5/site-packages/playhouse/pool.py", line 126, in _connect
    len(self._in_use) >= self.max_connections):
TypeError: unorderable types: int() >= str()
```

and
perhaps there are still other types of errors
