Fixes #1890.

Merge 3.6.x branch to master

Route "compiler" value of config_setting through @bazel_tools/tools/cpp:compiler.

This performs more testing for field descriptors built from byte
strings too, but that's mostly incidental. The chief intent is to
check that cross-linking occurs.

On Windows invoke plugins using cmd.exe

update to Guava 20

Python protobuf has long been a single-source codebase. 2to3 shouldn't need to run in setup.py.

Upport kokoro/release/python from 3.6.x branch

Down-integrate from google3.

Add experimental collect_all_artifacts script

Update README.md with gradle instructions for java

* Fix conformance running nothing issue

This change adds a source dependency of the test suite implementaion
class in the main function. For generality reason, the main function is
moved to the file of the test suite implemetation.  New test suite
implementation will need to implement the main function.
In order to make it easy for test suite implementation to implement the
main function, this change also refactor out the common code out of the
main function.

* Fix typo

This will find .bat files as well as .exe.

Only affects the case when plugins are invoked from PATH.

This does not change behaviour when compiled under Cygwin..

Added conformance test impl file to CMake.

Fixes JSON Loading of Any messages with Unknown Fields  in C++

Added cmake build files for the conformance test runner.

Add contributing guidelines.

* Fix well known type class not inheriting Message

Previously, well known types only implement Message interface but not inhert it.
Fix the problem by calling zend_do_inheritance

* Add back commented tests

* Add missing reference to conformanc_test_impl.cc in Makefile

* Update conformance failure list

Credit goes to @mercret for the fix.

Use ./tests.sh cpp to run it

update version

Request new option extension (and update URLs for existing projects)

Update options.md | fix website url for C# port