Fix boundary constraints when using percentage-based width/height (also on Firefox)

The last solution provided by @oslego does not work on Firefox, at least in version 21.0 on linux.

Here is a fix, which consist of:
* calculating the width/height from the parent node in case clientWidth/clientHeight is invalid
* replacing paper.width, paper.height by getPaperWidth(), getPaperHeight()
* updating boundary on drag start events (because the width/height may change)

Fixes #83 also on FF.