Added possibility to directly configure the gelf publisher

[Spea]

This PR adds the possibility to configure the publisher for the gelf handler directly (as discussed in #6).

The following examples are valid configurations.

monolog:
    handlers:
        gelf:
            type: gelf
            publisher:
                hostname: localhost # required
                port: 12201 # optional
                chunk_size: 1420 # optional

You can optionally define a service id so you can use the publisher wherever you want.

monolog:
    handlers:
        gelf:
            type: gelf
            publisher:
                id: my.publisher.id
                hostname: localhost # required
                port: 12201 # optional
                chunk_size: 1420 # optional

However, it is still possible to use your own defined publisher service.

monolog:
    handlers:
        gelf:
            type: gelf
            publisher: my.publisher.service_id

# or

monolog:
    handlers:
        gelf:
            type: gelf
            publisher:
                id: my.publisher.service_id

[stof]

This message is wrong. What must be set is either the hostname or the id

[Spea]

Fixed it :)

[Seldaek]

Looks fine to me now. Thanks!