v0.4.0 Release.

Bigtable

• More experimental asynchronous APIs, note that we expect to change all these
  experimental APIs as described in #1543.
• Most of the admin operations now have asynchronous APIs.
• All asynchronous APIs in noex::* return an object through which applications
  can request cancellation of pending requests.
• Prototype asynchronous APIs returning a google::cloud::future<T>,
  applications can attach callbacks and/or block on a
  google::cloud::future<T>.

Storage

• Use resumable uploads for large files in Client::UploadFile().
• Implement support for the userIp optional query parameter.
• BREAKING CHANGE Client::RewriteObject(), Client::CopyObject(), and
  Client::ComposeObject no longer require the ObjectMetadata argument.
  Instead use WithObjectMetadata(), which can be omitted if you do not need
  to set any metadata attributes in the new object.
• When using OpenSSL-1.0.2 the client library needs to configure the
  locking callbacks
  for OpenSSL. However, the application may disable this behavior if the
  application developer is going to use their own locking callbacks.
• When refreshing OAuth2 access tokens the client library uses the same retry
  and backoff policies as used for the request itself.
• Applications can set object metadata attributes via the WithObjectMetadata
  optional argument to Client::InsertObjectMedia().
• Applications can configure the library to only retry idempotent operations.
• The client library can use Google Compute Engine credentials to access the
  service.

Common

• Implement google::cloud::future<T> and google::cloud::promise<T> based on ISO/IEC TS 19571:2016, the "C++ Extensions for Concurrency" technical specification, also known as "futures with continuations".