Added a basic PSR-3 and RFC compatible logger

This patch is basically a shim which makes drush_log() compatible with the new Drupal 8 logging system.

It defines a PSR-3 compatible logger class which sends output to drush_log(). It does this alongside existing loggers, so messages will also be logged to the database or syslog (if those loggers are enabled).

I haven't really tested how it works with drupal_set_message(), which still uses the session (not the logging system) to store messages. This logger doesn't touch the session at all, so I don't think it will catch messages set with drupal_set_message(). But it seems like Drush has a separate mechanism for that.

So, it's a start. At the very least, this should allow dependency-injected code to output messages to the drush shell without calling drush_log directly.

EDIT: This partially addresses #883.