require('toobusy').maxLag(10) does not return a function

When using:

```
var toobusy = require('toobusy').maxLag(10);
```

`toobusy` is not a function. The right way seems to be:

```
var toobusy = require('toobusy');
toobusy.maxLag(10)
```

Therefore, the README is misleading IMO.
