adds fix for FF/IE scroll sync

This is a fix for a bug happening with gulp in the ghostMode when you open FF, Chrome and IE side by side. When you try to scroll a page in Chrome: both Firefox and IE just jump to the top ( like they got ```scrollTo(0)``` ). Same happens if you do it the other way round, basically breaking the whole scroll sync feature. I figured out that ```dBody.scrollHeight``` works best in webkit, other engines have significant problems getting this value right, therefore the calculation is off, causing  ```scrollTo(0)```. Found a good solution, props in the code comments, that fixes the issue and makes all the browsers sync scroll position again. The only problem is that ```documentElement.scrollHeight``` doesn't get proper value when running in karma. I guess that the test should be rewritten to reflect that inconsistency firefox is producing (for example when you pass ```scrollHeight: 1000``` the value it reads is around 1431). Besides that, everything seams to be working fine.