Fix JS exception in IE due to router inheritance

This should fix #121. In IE, there was a JS error thrown by the
`BaseRouter`'s constructor. It turns out it's just an issue with the way
the example app sets up prototypical inheritance -- IE doesn't support
setting `Router.prototype.__proto__ = BaseClientRouter.prototype`, so we
use `Object.create()` instead, which works in IE due to the polyfill
provided by `es5shim`/`es5sham`.
Merge pull request #152 from airbnb/fix-router-ie-error

Fix JS exception in IE due to router inheritance