Added server feature spec to echo metadata.
Clarified client spec for testing metadata.
Removed Status for tests that are now ready for implementation.
Modified the spec for UnimplementedCall to take an Empty request.

- test is similar to the auth tests
- uses an auth library to obtain the auth token OOB. This should be simplest, as many languages already use such an auth library in their existing auth tests.

Defined server side behavior needed to echo status code and message . Assorted minor fixes to spelling.

custom_metadata, status_code_and_message and unimplemented_method are now ready for implementation and blocking beta release.

This also removes some obsolete discussion of what Stubby does/did; this
was useful for contrasting but isn't particularly interesting now.

This adds both jitter and explicit values for the "proposed" algorithm
that implementations should use.

This is a faithful representation of the current version of the Google
Doc with only formatting changes.

This is a faithful representation of the current version of the Google
Doc with only formatting changes.

resolve comments

It is a partially manual port from Google Docs to Markdown. The content
was not changed in any important way.