Objc increase test coverage

Add Xcode 9 support to the helper script.

ObjC: Fix merging of length delimited unknown fields.

- Add a test to cover this and tweak the test to not use two merge
  paths to be sure things are as expected.

* Use constexpr more with VC++ 2017

Chrome's official builds have over 170 dynamic initializers for
variables of the form *::TableStruct::aux. Defining
PROTOBUF_CONSTEXPR_VAR to be constexpr for VS 2017 gets rid of all of
these and saves about 10 KB of binary size.

* Update generated_message_table_driven.h

Restore accidentally deleted line.

generate python code when calling PROTOBUF_GENERATE_PYTHON

* Enforce all error report for php tests.

* Import vendor/autoload.php in tests/bootstrap_phpunit.php

protobuf_generate: add EXPORT_MACRO option

Test Shutdown can be called multiple times.

Allow initializing a chain of protos using only a hash in Ruby

* Add $ before url_prefix_len to make it a variable.

* Remove unnecessary string length check

* Remove useless $type_url_len

Fixed to_h with repeated messages to return hashes in Ruby

Conformance Tests for Null Timestamp and Duration JSON Values

Integrated internal changes from Google

Fix go example test.

It needs "-I" flag to find imported file google/protobuf/timestamp.proto

Clang warning cleanup for unused parameter.

Fix policy warning CMP0054

Google.Protobuf should target net45 (upport to upstream/master)

fix protobuf_generate function

Add bazel support for examples.

This addressbook.proto now belongs to its own bazel pacakge and can't be
accessed in bazel protobuf_test target.

The example utilizes native bazel rules (proto_library, cc_proto_library,
java_proto_library, java_lite_proto_library) to show how easy it is to
build protobuf with bazel's native support. It also makes use of well
known types which was not possible until the latest bazel 0.5.4 release
and https://github.com/google/protobuf/pull/3594 .