* Prevents installation of `phpunit/phpunit:^6` because the test cases in this project require `\PHPUnit_Framework_TestCase` which was removed in v6. Moves generated PHP classes for tests into autoload-dev so that they are not referenced in the dist autoload. Adds Composer scripts to run protoc with and without grpc.

* update composer test target to use the same commands as generate_php_test_proto in tests.sh

* 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

* Follows proper autoloading standards
 - Splits PHP classes in descriptor.php into separate files
 - Splits MapFieldIter and RepeatedFieldIter into separate files
 - Moves descriptor.php to Internal/functions.php
 - Moves all namespaced functions into Iternal/functions.php

* fixes Makefile.am for added php files

* [PHP] moves all functions to GPBUtil

* removes description.php from the makefile

This option will be prepended to generated classes of all messages in
the containing file.

1. Generate single file for each message.
2. Lazily initiate metadata.