[PHP5] First namespace draft

I'm making this pull request here, [because how suggest by](https://github.com/phpseclib/phpseclib-php5/issues/1#issuecomment-27766802) @bantu, we should use one repository for everything. Would be nice if someone could create a `dev` branch where we can place all the new php5 stuff.

This pull requests adds basic namespace support. Tests are failing because of constant not autoloaded, but this should by merged in a dev branch anyway.