Deprecate MySQL driver based on deprecated PHP mysql extension

In PHP 5.5, support for the original `mysql` extension [was deprecated](http://php.net/manual/en/migration55.deprecated.php).  This affects our base MySQL driver class as it implements this extension.  This PR will tag that class, and its supporting classes, as deprecated since PHP will presumably drop support for them at some point.

Future TODO - When PHP announces a version where this extension will be removed or become totally unsupported, the driver class should be updated to fail gracefully (through version checks and Exceptions) instead of throwing a fatal error due to the code not being present.