Removing sniffing in Drag.Move, more info in the PR

Removed: 
```javascript
top += ((Browser.ie6 || Browser.ie7) ? 0 : containerMargin.top) + offsetParentPadding.top;
```
Replaced with:
```javascript
top += offsetParent == document.body? 0 : containerMargin.top + offsetParentPadding.top;
if (top < 0) top = 0;
```

When actually testing for this we discover that the bug was present in current Chrome, by looking at the code and testing on different browser I manage to fix the bug that @arian made, basically setting the top to 0 if the container is the body and to limit to 0 so top is never negative (this happens in IE7).

If you find different cases where the original bug is triggered just update this PR and I'll amend accordingly