Adds support for trailing metadata

- removes the status class, replacing it with a Struct
- adds support for trailing metadata, merging into the call's initial metadata

- tracks []
	Change on 2014/12/15 by temiola <temiola@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82193372