Add support for Constellio and custom proxy / user-agent

Constellio is a custom Apache Solr admin / client interface.

As Constellio has the concept of collections (basically an index with custom configuration parameters), the XML web service need a new query string parameter named collectionName. The new CollectionParameters class can be used to specify the desired collection:
            var resultats = solr.Query(new SolrQuery(this.searchTerm.Text), new QueryOptions {
                Collection = new CollectionParameters {
                    Name = "newcollection",
                },
            });

As we are behind a corporate firewall, we also need to specify a custom proxy and user-agent to use cloud solutions. The proxy / user-agent can be modified in the initialisation process:
            IWebProxy proxy = WebRequest.DefaultWebProxy;
            proxy.Credentials = CredentialCache.DefaultCredentials;
            string userAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.154 Safari/537.36";
            Startup.Init<Resultat>("https://rev01.cloud.constellio.com/constellio/app", proxy, userAgent);