0.5.0

Features

• Add ServiceAccountSigner interface and implement signing capabilities (byte[] sign(byte[])) in AppEngine and service account credentials (#73)
• GoogleCredentials.getApplicationDefault no longer requires google-auth-library-appengine to be in the classpath to create AppEngine credentials. AE SDK is accessed using reflection (#71)
• Make credentials serializable. Add HttpTransportFactory class to wrap HttpTransport creation an enable serialization (#69)
• Add asynchronous getRequestMetadata(URI, Executor, RequestMetadataCallback) to Credentials to allow getting request metadata without blocking (#64, #65)
• Check CLOUDSDK_CONFIG for default credentials location (#63)

Many updates

This will fix a problem with 2LO tokens expiring after an hour.