Rename mollom.class.inc into Mollom.php.

Some frameworks use autoloaders that discover all OO code by scanning the filesystem for .php files.

Putting this up for review, as it affects all current client implementations.

Lastly, note that while the new filename is Mollom.php, the code does not use PHP namespaces yet.  That is, because namespaces are PHP >=5.3.0 only.  (It is, however, very certain that there's going to be a PHP 5.3 version of the Mollom client class leveraging namespaces in the future.)