Added php docblock comments and changed constants to use newly created constant classes
Constants.php
 * Removed file

Api.php
 * New class made for Api-related constants

ExceptionMessages.php
 * New class made for Exception Messages constants

Essentially, just moved the constants out into their own classes. Using classes so that it isn't necessary to do an include/require on any other files, just import the class.
Updated constants based on constant class changes. Also added php docblocks to all functions and member variables.
Changed class docblock and added a link to the readme file with instructions
* Changed class docblock to include link to readme for instructions.
* Added docblocks for functions and member variables.
* Updated constants to use constants classes
* Changed finalize() - removing the return (since the sendMessage() method already does not return anything)
* Added php docblocks to functions and member variables.
* Changed constants to use new constant classes.
* Changed class docblock to include link to readme file
* Moved function descriptions of sendMessage() and verifyWebhookSignature() methods to function docblocks
* Removed unnecessary constructor
* Added function docblocks
* Changed class docblock to include link to lists readme