Add "getModifierState" to the MouseEvent interface

All browsers excluding IE8 implements this, I chose not to polyfill it for IE8 as it would only support the modifier keys already exposed by the event itself (which is also the traditional way of doing it), the real benefit of `getModifierState` is that it support *Lock-keys too, which we cannot detect. So polyfilling `getModifierState` would only hide the fact that it's not detectable. There's still some value to doing it, but I feel that an actual use-case is necessary first to determine the best course of action.

Related to and *duplicated* in https://github.com/facebook/react/pull/776, but not in any way dependent on.