protoc-gen-php: Support for package namespaces

Been working on rudimentary support for package based namespaces.

It needs some additional work before it should/could be merged, but I thought I would post it anyway. Especially the following would be nice:
 - [PSR-0](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md) compliance
 - Support for nested types with namespaces instead of underscores (almost requires PSR-0)
 - A way to disable the feature via (php) option or otherwise

It works though, so if you really need/want namespaces it's usable.