Cherry pick objc changes into 3.0.0-GA branch

If a message is proto3, then the zero values still count as being set one the
field is in a oneof.

Add tests to confirm oneofs work as expected in both syntaxes.

Fixes https://github.com/google/protobuf/issues/1933

Add a new test that forces strings into two different implementations from the
NSString class cluster to help confirm we're exercising both paths by which
CodedOutputStream will extract data from an NSString.

Move the old +load test (that was flawed because the behavior really depends on
the type of string from the NSString class cluster); into a unittest that
targets the specific case we're adding a behavior confirmation on.

As a bonus, improve the TextFormat generation of string characters < 0x20.

Fixes extra whitespace on generated comments.

Convert mapping of proto comments to appledoc format so they show up in Xcode and cocoadocs.

Fixes https://github.com/google/protobuf/issues/1866

Work for #1866 

Migrates all the public class docs over to appledoc format.  While Xcode is fine with blank lines in `///` comments, appledoc (used by cocoadocs) isn't and was leaving a bunch of info off the doc pages.

The generator still needs to be updated to do this also; that will be a follow up CL.

Add and fix C++ runtime docs

Add python/setup.cfg into dist files

Run Java compatibility tests on Travis.

The test is testing the binary/source compatibility between 3.0.0-beta-4
and the current version (right now it's 3.0.0).

Change-Id: Ic410b535758b38cee7baa99a15a0433734df44e6

add setup.cfg for building wheels

Fix the build-zip.sh to add .exe for win packages.

Change the build.zip.sh to support lite

Fix build protoc script for windows

Change log for 3.0 GA release.

Make protoc-artifacts able to build plugins.

Fixes traivs cpp build.

Remove the use of INT64_C/UINT64_C and add static_cast when neccessary.

GA fixes

Bump version number for GA

We need to pick up some of the last-minute Ruby fixes.

Ruby: translate package names from snake_case -> PascalCase.

Add a script to build protoc zip packages.

[ci skip]

Change-Id: Ia0e0bf3bb6bd24633dc8f294318bb20ac837f8b9