Add missing user-agent export (#75)

lib/msRestAzure.ts

@@ -1,7 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License. See License.txt in the project root for license information.
 
-export { AzureServiceClient, AzureServiceClientOptions } from "./azureServiceClient";
+export { AzureServiceClient, AzureServiceClientOptions, getDefaultUserAgentValue } from "./azureServiceClient";
 export { BaseResource, BaseResourceMapper } from "./baseResource";
 export { CloudError, CloudErrorMapper } from "./cloudError";
 export { CognitiveServicesCredentials } from "./credentials/cognitiveServicesCredentials";