Add PSR-3 logging support to modX

Restructure modX::_log() to dispatch to a PSR-3 logger when one is
registered via the new setLogger() method. The PSR-3 logger replaces
legacy FILE/ECHO/HTML output while modRegister logging is preserved.

Includes 20 tests: 11 baseline tests locking down existing legacy
logging behavior, and 9 tests covering PSR-3 integration.

Author: pbowyer