History singletons

[Schniz]

Instead of just updating the docs, this is an implementation for what @ryanflorence said on #1339 and @mjackson on #1426.

this PR lets you use a history singleton with BrowserHistory and HashHistory:

var { history } = require('react-router/lib/HashHistory');
React.render(<Router history={ history } ... />, ...);

[Schniz]

@mjackson, I decided on calling it history instead of getSingleton.
Its less about implementation detail (that can be changed later), and more about what you get from this function

[mjackson]

People who need to support IE8 (poor folks) are going to gripe about this. Let's just export var history = new BrowserHistory in this module instead. Then people can still import { history } and we'll be fine.

[Schniz]

👍

[mjackson]

Thanks for your work on this @Schniz !

[gaearon]

💃

[Schniz]

damn you IE 8.

[Schniz]

@mjackson fixed this 👍

[Schniz]

the tests don't pass for some reason, and don't seem to be mine. should I rebase for the current master?

[mjackson]

@Schniz The only failing test is the one that tests scrolling, which @taurose fixed in c61a0f5. We're currently working on getting that branch merged, so this one should be good to go for now. When that branch merges we'll be green again.

Thanks!