Less dependencies
I see in dependencies `Symfony/*` and `guzzle/http` (they are 66% of Pawl dependencies).
Especially Symfony dependencies with all tests and docs are far from lightweight.

> I'm not using Symfony components, not using sessions or routing provided by Ratchet, but I do use Pawl (usage is the same as example in repo).
> Any suggestions what can I safely remove without loosing everything else?

In fact it would be great to move sessions and routing into separate component for those who do not need them.

P.S. I know, there are people who just run `composer install` and do not care about what is inside, but I do care, and want to have the smallest possible set of dependencies.
