Added support for the GelfHandler

[Spea]

This PR adds support for the GelfHandler which was added in Monolog 1.1.0

[Seldaek]

Shouldn't we auto-configure a publisher too if the php-gelf classes are available?

[Spea]

Maybe this is a good idea, yeah. I could make the publisher node a variable node so the handler can be configured like this:

monolog:
    handlers:
        gelf:
            type: gelf
            publisher:
                hostname: localhost # required
                port: 12201 # optional
                chunk_size: 1420 # optional
            level: DEBUG
            bubble: true

This will still work

monolog:
    handlers:
        gelf:
            type: gelf
            publisher: my.publisher.id
            level: DEBUG
            bubble: true

What do you think about this?

[Seldaek]

Sounds good to me, I just don't think we should force people to define services for simple usage, because that's quite a high barrier for many newcomers.