Fix Drag when no offsetParent

Upgraded my largest project to current code (1.5.1-dev) and found a issue I created with the last commit.
While fixing this found also a better logic in the code to be more localized in the Class. 

- Fix Drag when no offsetParent
- default to false the new "compensateScroll" option
- add it in the docs

[Fixed jsFiddle](http://jsfiddle.net/Sergio_fiddle/asn2j0xg/) - drag follows with scroll
[Broken jsFiddle](http://jsfiddle.net/Sergio_fiddle/asn2j0xg/1/) - drag do not follow with scroll
Isn't this basically always true? If `z` is in `modifiers`, why should it be falsy?