Xcode 8.3.x cleanups and use on travis

Add python 2.6 test back for cpp implementation. 

Make .bzl files compatible with future versions of Bazel

- Xcode 8.3.3 bumped the simulators to 10.3.1, so 10.3 fails.  Just use
  the "latest" naming instead to avoid issues.
- Also switch to just testing an old 32bit build and new 64bit build as
  there isn't any code in protobuf that should care about form facts.
  (Looks like the iPad Pro simulator names aren't always constantly, on
  some developer machines they are "iPad Pro (9.7 inch)" while on others
  they are "iPad Pro (9.7-inch)"; so just avoid using them.)

* Fix the bug in php c extension that setting one field can change another
field's value.

The reason is that previously, in c extension, it was assumed that the
order that fields were declared in php is the same as the order of
fields in upb. This is not true. Now, for every field in upb, we will
look up the actual property that is corresponding to the upb field.

* Cleanup pull request

* Fix indentation

* Port to php5

* Port with php7.1

* Port to zts

* Add descriptors test

* Update descriptors tests

* Add public descriptors

* Add test_desriptors.proto to test script

* Update composer files

* Remove references to GPBType, update tests to be compatible with c

* Update for c extension compatibility

* Remove nested enums for descriptor, update tests

* Strip leading '.' from descriptor name

* Update tests with test for getClass, fix OneofDescriptor

* Add new files to Makefile.am

1, Some tests in reflection_test PY26 raise TypeError but other versions raise ValueError for convert negative long to unsigned
2, Change compare exception type to compare exception str for testDuplicateExtensionNumber. Original code raise 'Double registration of Extensions' is not an instance of (<type 'exceptions.AssertionError'>, <type 'exceptions.ValueError'>) for PY26 cpp implementation

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Update third party addons with ProfaneDB

I added my project ProfaneDB, it is a database for Protocol Buffers objects. Written in C++, it uses gRPC as an interface for other languages.
It is still work in progress, but I'd love to get some feedback on it while I progress!

divideInt64ToInt32 is called statically from protobuf/php/src/Google/Protobuf/Internal/CodedOutputStream.php
(the only reference)
This causes fatal error in PHP 7.1 (32-bit only because 64-bit doesn't use this function)

Add classpath for java example Makefile

Update the comment on the message_type to cover what it should be.

Objc proto2 conformance test support

- Better error message for unknown messageType.
- Remove unneeded if.

Both test_messages_proto3.proto & test_messages_proto2.proto define
message ForeignMessage {...} and enum ForeignEnum {...} but since
objc doesn't use the proto package in the naming, these end up
conflicting.

Adding the objc_class_prefix option to the proto3 file ensure the generated
objc types are all unique.

Fix issue #1745 - javascript allow dot in filename

Add declaration for AddDescriptorsImpl()

AddDescriptorsImpl() is a private static implementation detail,
and needs to be declared but won't be used from outside the
source file.  Place it inside an anonymous namespace to fix a
previous build error that would result in more restrictive
build enviornments that use -Werror=missing-declarations or
similar compiler flags.

Proto2 test message support to conformance test

Windows: support long paths in open/mkdir/access

Treat missing declarations as an error for test builds, to prevent
future updates which add functions without properly declaring them.
This will prevent broken builds in more restrictive build environments.

CMake: Install .pc Files

Exclude the unconfigured .pc input files of the CMake
based install: they shall not be packed/distributed,
only their outputs are