Code should exit if MutationObserver isn't supported.

[bryanbraun]

The polyfill relies on MutationObserver which isn't supported in several environments (like old IE). That's not a big deal since those environments don't typically have the 300ms delay, but we need to exit early if the MutationObserver isn't supported or else we'll get errors in those environments. For example:

IE 9:

PhantomJS 1.9.8:

[arschmitz]

@bryanbraun good catch thank you will get this fixed and thank you for the other 2 patches!