Implement OC\Log as proxy to OC_Log

OC\Log implements the Psr\Log\LoggerInterface interface. See
https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-3-logger-interface.md
for the full interface specification.
Fix OC\Log with OC_Log in wrong namespace
Remove include for loading legacy class
Code style
Use the constants from OC_Log
Remove default return hint
Make the object drive the logging backend

This is the other way around then it was.
Fix review points
missing $
Fix usage of non existent consts
Merge pull request #3268 from owncloud/convert-oc_log

Convert OC_Log to object so it can be used for DI