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GCS v1.0.0

Declared GA and updated major number.
Support signed policy documents.
Support service account key files in PKCS#12 format (aka .p12).
Support signing URLs and policy documents using the SignBlob API, this is
useful when using the default service account in GCE to sign URLs and policy
documents.

Breaking Changes
- Return google::cloud::future from InstanceAdmin functions: this is
  more consistent with all other functions returning futures.
- Remove unused bigtable::GrpcError: the library no longer raises this
  exception, any code trying to catch the exception should be modified to
  handle errors via StatusOr<T>.
- Remove Snapshot-related functions, tests, examples, etc.: this is
  whitelisted functionality in Cloud Bigtable and it is no longer expected
  to reach GA.
Continue to implement more async APIs (Async*() methods) for the
InstanceAdmin, TableAdmin, and Table classes
(Note: These are not yet stable)
Bugfixes:
- Need ignore_warnings to actually delete an AppProfile.
- Fix portability/logical errors in shell scripts.
- Fix a race condition in MutationBatcher.
Implemented a number of previously missing code samples.