The using directive in headers

[Bart Cerneels]

I've been stuck with a build error all morning concerning conflicting
classnames.

Since "using namespace Meta" is in so much of our headers the Meta
namespace basically becomes useless. If one included header, even
recursively, has this. Every class with the same name as a Meta::
class would result in ambiguous name errors.

Currently failing: Playlist, PodcastEpisode, PodcastChannel, etc.

So please refrain from utilizing the naming directive in headers.

Stecchino