Guzzle 6 adapter

[ddeboer]

Could you create a guzzle6-adapter repo? I’d be willing to work on the code.

[sagikazarmark]

I've created a repository with some basic content, but please keep the following in mind:

• The interfaces are not finished (yet). Guzzle 5 is the example implementation
• Currently I am working on some separation logic, see the internal_separation branches in the relevant repositories

I've also added the package to packagist, travis and scrutinizer