fix Firefox ignoring target margins

Thank you so much for creating the zoomerang - I used it on my website: http://www.tknomad.com :+1: 

So, it turns out my Firefox (version 25.0) was reporting 'marginLeft' computed style, etc. but not 'margin'.  Chrome reports 'margin-left', etc. format, but FF does not.

Just give your test image margins and test in Firefox to see the bug - image transforms to wrong position on close() and then jumps to correct original position on end of transform.

Some dev screenshots:

![margin-ff-undefined](https://f.cloud.github.com/assets/451178/1722207/56d43e4a-623c-11e3-8a59-bc0a66f52b0a.png)
![margins-ff-undefined](https://f.cloud.github.com/assets/451178/1722209/5e770f1a-623c-11e3-97ff-02cb7932795c.png)
![margins-chrome](https://f.cloud.github.com/assets/451178/1722212/6dc6c104-623c-11e3-8d2d-5551b4abf5a7.png)

I read this doc:

http://javascript.info/tutorial/styles-and-classes-getcomputedstyle#style

And camel casing seems to do the trick.

Tested on:

* Firefox 25.0
* Chrome Canary 33.0.1734.5
* Chrome Beta 32.0.1700.41
* Safari 7.0 (9537.71)
* Opera Next 19.0.1326.9

And all is fine!

Next up, I'm getting funky effects on my Android Browser.  I might be troubleshooting that next.