added duScrollOffset default value option for du-smooth-scroll

Hey! Great work on this, I love the smooth scrolling option especially. I found that adding a value to the module allowed me to set the offset once in my config section and made my HTML look a bit cleaner.
Yes, it could very well be 0 instead of null, I honestly don't know why I chose null. 
This code seems to rectify that anyway:
```js
container.scrollToElement(angular.element(target), isNaN(offset) ? 0 : offset, isNaN(duration) ? 0 : duration);
```
because `isNaN(null) === isNaN(0)`
so either one would be reassigned to 0 anyway.