Adding the ability to download build configs

Thanks to Mark deVilliers, we now have the ability to download build configurations. Sample usage would be as follows:

var client = new TeamCityClient("192.168.1.84:8111");

client.Connect("user", "user");

var config = client.BuildConfigs.ByConfigurationId("SignalRRabbitMQ_Build");
client.BuildConfigs.DownloadConfiguration(BuildTypeLocator.WithId(config.Id), Download);

Also in this release is a small change of semantics when connecting as guest. There used to be only 1 connect method that accepted the parameters name, password and a bool to suggest to connect as a guest. Thanks to a suggestion by Eugene Petrenko, the following is now implemented:

void Connect(string userName, string password);
void ConnectAsGuest();