[changed] fallback to window.location for history

When `<Routes location=“history”/>` is used
we now fall back to window.location for browsers
that don’t support the HTML5 history API

Rather than falling back to hash, falling back to
window.location ensures urls are identical for
all users, and sharing them will always work

closes #50
Merge pull request #126 from rackt/history-fallback

[changed] fallback to window.location for history
Updated docs for <Routes>
[added] initialPath prop to <Routes>

Fixes #111
[added] Router.AsyncState mixin

Helps with #57
Add note about returning hash from getInitialAsyncState