Can not Customize storage_driver

[wmzy]

As the doc, connect should return a bool. So the line https://github.com/pintsized/ledge/blob/2474cab721dc300b1dbc6ba40121283ba44bea47/lib/ledge.lua#L228 should be replaced by return driver, nil; And the bool new() on the doc should be change.

While I return nil(return nil for no max) on get_max_size method, here will throw an error(attempt to compare nil with number).