[added] Router.AsyncState mixin

Helps with #57
Add note about returning hash from getInitialAsyncState
Rename argument
Fixing error in updated example

The new example would throw an error because `Routes` is undefined.

It might be neater to use the new style with React 0.11.x in the examples, not sure how you feel about that. e.g.

var Router = require('router');
React.renderComponent((
  <Router.Routes>

...etc.
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
typo
Clarified docs
Avoid using window object in module scope
Revert "[added] initialPath prop to <Routes>"

This reverts commit b0aa491431a0fcf74120a9a24793f0131e6a4f88.
updated animations example to 0.5.x API
added docs/

- touched up some documentation
- added missing documentation

closes #128
fix docs
reflect new docs/ in contributing.md
move api/index -> api/README.md
typo