Adds directional tolerance option

Scrolling down you may want the header to pin immediately, while scrolling back up you may want to require a higher tolerance to avoid a jittery 'up/down' while making small movements. This allows for both a standard int value or a up/down options object.
Merge pull request #74 from chiplay/directional-tolerance

Adds directional tolerance option. Can be supplied by passing an object as tolerance:

```js
var hr = new Headroom(element, {
  tolerance : { down : 0, up : 20 }
});
```

If a number is supplied, it is normalised into an object where the up and down tolerance values are identical. 

```js
// this is equivalent...
var hr = new Headroom(element, {
  tolerance : { down 5, up : 5 }
});

// ...to this
var hr = new Headroom(element, {
  tolerance : 5
});
```