Added build number defined
Moved metabox constants to the metabox class
Moved metabox javascript to the javascript file
Added various comments
Updated enqueue functions to use version number and build number
Fixed all the const calls to use self::
Added check to all the function to prevent use of them directly before calling init()
Changed direct access to the $_GET and $_POST to use filter_input instead
Simplified save function
Updated the show metabox code to use some inline php to allow for easier code review