Added support for zoom factor in links

* Added support for zoom factor in links
* Moved stray method into plugin scope
* Use page object reference instead of pageNumber for destination. (wider
compatibility)

Please test in Reader, as there seems to be bugs in both Chrome and Firefox.

http://htmlpreview.github.io/?https://github.com/Flamenco/jsPDF/blob/master/test/test_annotation.html.