Fix zoom and pan issues (fix:#2 fix:#3)

Hi @mephux, @nemoguardx  and @bumbu, this is a PR about issue #2 (and maybe #3 too).

Maybe, you should use a less "square" layout (for both nodes and canvas) to test this plugin:  some of the inconsistency are *hidden* when the layout is very a square (because this is a very specific case).

Here is an illustration:
  - [nerite/1 - The current implementation](http://jsbin.com/nerite/1/edit?html,output)
  - [nerite/2 - With this fix](http://jsbin.com/nerite/2/edit?html,output)

Based on @mephux illustration :
  - [volageta/1 - The current implementation](http://jsbin.com/volageta/1/edit?html,output)
  - [volageta/9 - With this fix](http://jsbin.com/volageta/9/edit?html,output)