Add Guzzle4 driver

Make Guzzle4 driver to be used on REST server tests

This can be used by setting on `behat.yml` setting:
```
default:
    context:
        parameters:
            rest_driver: Guzzle4Driver
```

It's a simple upgrade from the GuzzleDriver (Guzzle v3)

This needs Guzzle4 on requirements added onto https://github.com/ezsystems/BehatBundle/pull/8