Throw Error when imported în non-browser environment

[asynxc]

Hello,

We use this module to polyfill IE10. But when using it in SSR powered application, it crash the server with following error :

var MutationObserver = window.MutationObserver ^ ReferenceError: window is not defined at Object.<anonymous> (/home/.../editor/node_modules/mutation-observer/index.js:3:24)

a PR submitted in order to fix this issue #10