* Implement a very simple prefetching solution.
* Remove next-prefetcher.
* Require 'whatwg-fetch' only in the client.
* Use xhr in the code.
* Use a simple fetching solution.
* Fix 404 and xhr status issue.
* Move the prefetching implementation to next/router.
* Add deprecated warnning for next/prefetch
* Run only 2 parellel prefetching request at a time.
* Change xhr to jsonPageRes.
* Improve the prefetching logic.
* Add unit tests covering the Router.prefetch()
* Update examples to use the new syntax.
* Update docs.
* Use execOnce() to manage warn printing.
* Remove prefetcher building from the flyfile.js
Because, we no longer use it.
