Updated API Link

Updated the API link to the latest version.
Linked afterFilter()

Added a link to the afterFilter() callback.
Added some links.

Added some links incase more information is needed about a topic.
Corrected a bad link.

Fixed a link that was directing to a 502 gateway error.
Link to latest API

A simple change that will always link to the latest API.
More API Link Fixes

Changes the API links to ensure they redirect to the latest API version.
Removed screencast.

I removed this screencast because it does not exist anymore.  I was
unable to find it on the cakePHP YouTube page.
Typo Fix

Simple typo fix in the format.
Application Extension Reference

Changed the link to a ref.
Fixed Broken Links

I fixed a bunch of broken links in the book and updated them to the 2.4
API.

I was not able to find a way to have the API website reroute the
requests to the latest API.  I figured working links are at least better
than broken ones for the time being.
Merge pull request #939 from ezrabynx/master

Added Links