[WIP] Add endpoint support

As said in #25 we add support for the new endpoint integration. The config remains what @skrivanos said in his PR plus the ```default_endpoint``` param and the Curl adapter by default to match with Solarium.

```
nelmio_solarium:
    endpoints:
        default:
            host: localhost
            port: 8983
            path: /solr
            core: ~
            timeout: 30
    clients:
        default:
            adapter_class: 'Solarium\Core\Client\Adapter\Curl'
            endpoints: [default]
            default_endpoint: default
```

This is a work in progress. Ideas for new tests or comments in general are welcome