Add a PSR-3 based logging interface

Add a PSR-3 based logging interface

The MWLogger class is actually a thin wrapper around any PSR-3
LoggerInterface implementation. Named MWLogger instances can be obtained
from the MWLogger::getInstance() static method. MWLogger expects a class
implementing the MWLoggerSpi interface to act as a factory for new
MWLogger instances. A concrete MWLoggerSpi implementation using the
Monolog library is also provided.

New classes introduced:
; MWLogger
: PSR-3 compatible logger that wraps any \Psr\Log\LoggerInterface
  implementation
; MWLoggerSpi
: Service provider interface for MWLogger factories
; MWLoggerNullSpi
: MWLoggerSpi for creating instances that discard all log events
; MWLoggerMonologSpi
: MWLoggerSpi for creating instances backed by the monolog logging library
; MWLoggerMonologHandler
: Monolog handler that replicates the udp2log and file logging
  functionality of wfErrorLog()
; MWLoggerMonologProcessor
: Monolog log processer that adds host:wfHostname() and wiki:wfWikiID()
  to all records

New globals introduced:
; $wgMWLoggerDefaultSpi
: Default service provider interface to use with MWLogger
; $wgMWLoggerMonologSpiConfig
: Configuration for MWLoggerMonologSpi describing how to configure the
  Monolog logger instances.

This change relies on the Composer managed Psr\Log and Monolog libraries
introduced in Ie667944.

Change-Id: I5c822995a181a38c844f4a13cb172297827e0031