* 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.