Merge pull request #4 from stanley-cheung/php_creds_api_change

src/php/ext/grpc/call_credentials.c

src/php/ext/grpc/call_credentials.h

src/php/ext/grpc/channel.c

src/php/ext/grpc/channel_credentials.c

src/php/ext/grpc/channel_credentials.h

src/php/ext/grpc/config.m4

src/php/ext/grpc/package.xml

src/php/ext/grpc/php_grpc.c

src/php/tests/interop/interop_client.php

src/php/tests/unit_tests/SecureEndToEndTest.php