PHP library and sample update.

The following is loosely the What's New:

Removed the .dll and the requirement to build the DLL in Windows. This makes this PHP sample now truly cross platform (runs on my Mac!)

Updated documentation for PHP Service Principal and tenant creation

Updated SimplePHP library with new AudienceURI and SPN formatting, as well as updated the Saml2TokenValidator.php for AAD (we'll need to push this up to SimpleSAML PHP for our AAD module as well)

refactored the library to be a *lot* simpler. This removed some library dependencies we had in the previous version.