[added] Router.renderRoutesToString

[fixed] Initial render to null causing second render
[fixed] ActiveState mixin causing second render
[added] Docs
[added] Tests

```js
Router.renderRoutesToString(routes, originalUrl).then(function (data) {
data.html; //string returned from React.renderComponentToString
}).catch(function (error){
//Redirect will set error.status and error.location
error.status; //302 for Redirect
error.location; //location to redirect to
});