Adding support for service account credentials.

- Tested end to end with a JSON key I generated for my account using the
fetch_oauth2 binary.
- The same fetch_oauth2 binary can get a token from the GCE metadata service on a VM in cloud.
	Change on 2014/12/19 by jboeuf <jboeuf@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82548689
8 files changed