Fix namespace for OCP\Appframework\Http

To avoid having to use OCP\Appframework\Http\Http in the public - and stable - API, OCP\Appframework\Http is now both a class and a namespace.
Basically the same method as used for Sabre VObject Property, Component etc except that Http has both a private and a public implementation.

Not strictly a bugfix, but OTOH urgent to have a stable and sensible API before releasing it.

@DeepDiver1975 @karlitschek