max_idle_time parsed as float

I frequently construct via:
```python
torndb.Connection(**re.search("mysql://(?P<user>\w+):(?P<password>\w*)@(?P<host>.*)/(?P<database>[\w\-]+)\?reconnect=true&max_idle_time=(?P<max_idle_time>\d+)",
    "mysq://user:pass@host/database?reconnect=true&max_idle_time=60").groupdict())
```
But `max_idle_time` gets parsed as a string, not a float. So adding the `float(max_idle_time)` will make this method possible.