[Proposal] Change to the psr loggerinterface. Related issue #658

Fixes #658 

Replaced \Symfony\Component\HttpKernel\Log\LoggerInterface with \Psr\Log\LoggerInterface.
Added psr/log to composer.json.

Symfony advises to use \Psr\Log\LoggerInterface instead of the \Symfony\Component\HttpKernel\Log\LoggerInterface. Since the symfony loggerInterface is deprecated and to be removed in 3.0.