Passes actual props to rendered Route.

Currently changed props are not passed down to children as reported by issue https://github.com/rackt/react-router/issues/268.

But this is necessary if the router should work with a functional cursor library, like:
https://github.com/mquan/cortex
https://github.com/dustingetz/react-cursor
https://github.com/caseywebdev/cursors

This pull request contains the changes necessary to support changing props. This was a quick way to get it working and might not be the prettiest, but I felt like I should atleast share this effort... Might help to integrate it into `computeHandlerProps` or so...