Add Composer managed libraries

This is the initial step towards implementing the Structured logging RFC
[0].

The Psr\Log library is declared as a requirement in the composer.json
file and Monolog is suggested as a package that can enhance MediaWiki.

To use this in a local checkout:
* Install composer [1]
* `composer update`
* `composer require monolog/monolog 1.9.*`
* `composer dump-autoload --optimize`
* `git update-index --assume-unchanged composer.json`

Additional work will be needed to provide both the Psr\Log and Monolog
libraries available for deployment to the Wikimedia Foundation servers.
The Psr\Log library will also need to be packaged in the release
tarballs.

[0]: https://www.mediawiki.org/wiki/Requests_for_comment/Structured_logging
[1]: https://getcomposer.org/doc/00-intro.md#installation-nix

See also: I1431b24 (Monolithic implementation)

Change-Id: Ie667944416187cfd2ae6016c9e2fa28f4204bcd7