Add php namespace support

Hi!

I've implemented php namespace support. Default behavior is the same as it was (with underscores) and to get classes generated with namespaces --use-namespaces parameter must be specified.
```
php protoc-php.php --use-namespaces some.proto
```
I know that option handling code in the script is not perfect but it works for now and maybe will be reimplemented if someone deside to add some more options to parser.