Defines a new Service class using a service identifier and list of versions
including an optional set of features (functions) to apply to the class
prototype.

@param serviceIdentifier [String] the identifier for the service
@param versions [Array<String>] a list of versions that work with this
service
@param features [Object] an object to attach to the prototype
@return [Class<Service>] the service class defined by this function.
function defineService(serviceIdentifier, versions, features) {
    if (!Array.isArray(versions)) {
      features = versions;
      versions = [];
    }

    var svc = inherit(AWS.Service, features || {});

    if (typeof serviceIdentifier === 'string') {
      AWS.Service.addVersions(svc, versions);

      var identifier = svc.serviceIdentifier || serviceIdentifier;
      svc.serviceIdentifier = identifier;
    } else { // defineService called with an API
      svc.prototype.api = serviceIdentifier;
      AWS.Service.defineMethods(svc);
    }

    return svc;
  }