Fix IE8 potentially throwing during restoreSelection
getModifierState incorrectly exposed as SyntheticMouseEvent.getEventModifierState
Fix small issue in ReactContext comment

Noticed this the other day and meant to fix it.
Remove _addPromise from Dispatcher code example 

`_addPromise` is not used in the provided Dispatcher example, nor does it exist in Dispatcher.js found here: https://github.com/facebook/react/blob/master/examples/todomvc-flux/js/dispatcher/Dispatcher.js
Merge pull request #1891 from robarnold/patch-1

Fix small issue in ReactContext comment
Merge pull request #1892 from thauburger/patch-1

Remove _addPromise from Dispatcher code example
Include Object.assign polyfill

This isn't a good final solution but it makes React actually usable on
its own.

This also makes tests runnable, though only via jest
(./node_modules/.bin/jest)
Simpler way to prevent flushing updates on server

Test Plan: Only failing tests in jest are immutable tests that were already failing.
Merge pull request #1908 from spicyj/simpler-server-willmount

Simpler way to prevent flushing updates on server
Move react-source gem out of project root

Now it requires a build step. At least the confusing lib directory is
gone though.
Merge pull request #1909 from zpao/move-react-source

Move react-source gem out of project root
Merge pull request #1879 from syranide/modistatewrong

getModifierState incorrectly exposed as SyntheticMouseEvent.getEventModifierState
Merge pull request #1774 from syranide/ie8focusthrow

Fix IE8 potentially throwing during restoreSelection