Specify animation duration on individual links

Adds support for data-menu-duration attribute

@@ -15,6 +15,7 @@
Add MENU_DURATION_ATTR variable

@@ -132,10 +133,18 @@
Check if MENU_DURATION_ATTR atrribute is set on link.
If so, set animationDuration to attribute value, else use _duration() function

Proposed Documentation:

Duration
-----

To specify a set duration for the animation on a specific link, use the `data-menu-duration` attribute. This allows for fine-grain control between presentation slides, for example, where one slide may have more animation than others.


```html
<a href="#target-element" data-menu-duration="2000">Link to target element</a>
```

[edit: updated markup example]

In this example, the transition to the #target-element anchor will take 2 seconds, over-riding the default.
```
		var animationDuration = _duration(_skrollrInstance.getScrollTop(), targetTop);

		if(menuDuration !== null) {
			animationDuration = parseInt(link.getAttribute(MENU_DURATION_ATTR));;
		}
```

Much cleaner. Fair point re: reusing var, though there are many instances in the code. I prefer 'use strict'; myself, but was trying to match style - although you have 'use strict'; on the self executing function (l.10), it doesn't seem to be obeyed as I haven't noticed it objecting. I would tidy it up, but I don't want to be making major changes like that.