Easing Implementation

Greetings,

Very nice plugin, I'll be using it for my site!

I added a jQuery Easing option for the scrollUp function, e.g.,
```
$.scrollUp({
  topSpeed: 600,
  easingType: 'easeInCirc'
});
```

It was just something I desired so I thought I'd share in case you'd like to implement.

I also noticed some spelling errors in your README so I fixed those as well.

Regards,
Allyson