Retry credentials on Compute Engine

Mitigate occasional failures in looking up Application Default Credentials on a Google Compute Engine (GCE) Virtual Machine (#110 addresses #109):

• Add retry to the credentials check for the GCE environment
• Use fixed IP address for the GCE VM metadata server used to perform the check (to avoid nameserver lookup)
• Allow skipping the check by setting an environment variable NO_GCE_CHECK=true
• Use a custom GCE metadata server address by setting the environment variable GCE_METADATA_HOST=<hostname>

0.7.0

• Add GoogleCredentials.createDelegated() method to allow using domain-wide delegation with service accounts (#102 addresses #90): see https://developers.google.com/identity/protocols/OAuth2ServiceAccount#delegatingauthority

• Retry HTTP errors in ServiceAccountCredentials.refreshAccessToken() to avoid propagating failures (#100 addresses #91)

• Allow bypassing App Engine credential check using environment variable, to allow Application Default Credentials to detect GCE when running on GAE Flex (#103)

Better token refresh

1. Refresh Auth token with 5 minutes left (was 1 min)
2. Several typo are fixed
3. GAE API's and Servlets are provided - so mark them that way.

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.