v0.3.0 Release.

This is the v0.3.0 release of the Google Cloud C++ Client Libraries.

Bigtable

• Experimental asynchronous APIs.
• Include an example that illustrates how to use OpenCensus and the Cloud
  Bigtable C++ client library.
• Several cleanups around dependency management with CMake.
• Jason Zaman contributed improvements and fixes to support soversion numbers
  with CMake.
• Lots of improvements to the code coverage in the examples and tests.
• Fixed multiple documentation issues, including a much better landing page
  in the Doxygen documentation.

Storage

• Automatically compute MD5 hashes and CRC32C checksums when objects are
  uploaded and downloaded. Any hash or checksum mismatched results in an
  exception. Applications can MD5 hashes, CRC32C checksums or both on any
  request.
• Parse Bucket lock and retention policy attributes in object and bucket
  metadata.
• Add APIs to upload and download files with a single function call.
• Improved the error messages generated when the credentials file is missing
  or has invalid contents.
• Jason Zaman contributed improvements and fixes to support soversion numbers
  with CMake.

Common

• google::cloud::optional<T> an intentionally incomplete implementation of
  std::optional<T> to support C++11 and C++14 users.
• Applications can configure google::cloud::LogSink to enable logging in some
  of the libraries and to redirect the logs to their preferred destination.
  The libraries do not enable any logging by default, not even to stderr.
• google::cloud::SetTerminateHandler() allows applications compiled without
  exceptions, but using the APIs that rely on exceptions to report errors, to
  configure how the application terminates when an unrecoverable error is
  detected by the libraries.