Switch Midgard API to use namespaces

[bergie]

Midgard3 should be built with modern PHP in mind, and with the idea of being as close to regular Midgard API as possible. Midgard's GIR and Python APIs are already namespaces-based, and so PHP should be too.

Instead of:

$qb = new midgard_query_builder('midgard_article');
$article = new midgard_article();

We should have:

$qb = new Midgard\QueryBuilder('midgard_article');
$article = new Midgard\MgdSchema\midgard_article();

[indeyets]

related mrfc: http://www.midgard-project.org/development/mrfc/0049/