Added PSR Handler

I have an application into which I need to introduce Monolog in order to use some of its other handlers, but I have an existing PSR-3 compliant logger class that I do not wish to change. Rather than port all of the formatting/processing/writing logic from my logger to match patterns established by Monolog, I'd like to simply tell Monolog to use my logger - just pass it whatever information it knows how to handle.

This handler allows anyone integrating Monolog in their application to continue using their existing PSR-3 compliant logger(s), if any, in addition to the variety of other handlers that Monolog makes available. The handler simply proxies to the `Psr\Log\LoggerInterface` log methods, which are the all lowercased versions of the standard Monolog severity levels.