Move towards PSR-3 compatibility.

See also #6.
Add a compatibility layer for PSR-3 logging.

This adds the \Plop\Psr3Logger class which provides a thin compatibility
layer with the PSR-3 LoggerInterface.
Hence, Plop now fully supports that PSR :-)

Fixes #6.
Update travis-ci configuration.
(experimental) Enable hhvm in travis-ci config.