[added] Router.AsyncState mixin

Helps with #57
Add note about returning hash from getInitialAsyncState
Rename argument
Merge pull request #127 from JedWatson/patch-1

Fixing error in updated example
Merge pull request #125 from rackt/async-state

[added] Router.AsyncState mixin
added upgrade guide
added link to upgrade guide
use exact versions in package.json

closes #130
[added] ScrollsOnTransition mixin

When a page’s content is long and you transition
to a new route, the scroll position persists
and its super dumb.

This mixin goes on a route handler that you know
has long content and scrolls the window up when
you leave the page.

Not a bad idea to put this on every route handler

closes #121