Add support for n arguments to `run`

Previously, `run` (and the implementation to be wrapped by levee) only accepted a single context argument and callback function. This change allows n arguments in addition to the callback, e.g.:

```javascript
var circuit = levee.createBreaker(function (foo, bar, baz, cb) {
    // use foo, bar, and baz
    cb();
});

circuit.run('foo', 'bar', 'baz', function (err) {
   console.log(err || 'ok');
});
```