Checking for MutationObserver not in global

At the moment the scheduler checks MutationObserver and then assigning global.MutationObserver - causes a lot of **fun** in IE 11 extensions.

Also, @stefanpenner warned me IE10 does strange things when using MutationObserver:

http://codeforhire.com/2013/09/21/setimmediate-and-messagechannel-broken-on-internet-explorer-10/

https://github.com/cujojs/when/issues/197

To be fair, I haven't run into any issues myself running promises on the page, but then again I did not run it with spin.js - I'll try to reproduce