Adding identity samples

Partially addresses #422 
The goal of these samples is to let a user of the SDK download any of them and execute them with the least amount of work. To that end, I would split off each of these three cases (either by username, ID or email) into their own sample files such as `delete_user_using_username.php`, `delete_user_using_id.php`, and `delete_user_using_email.php`.

Alternatively, you could introduce a set of three sample files on retrieving a user such as `get_user_using_username.php`, `get_user_using_id.php`, and `get_user_using_email.php` showing the three methods to retrieve a user. Then in this sample file (`delete_user.php`), just use one of those three methods.