Variant::convert() now also tries QVariant custom conversion (Qt5.2)

Since Qt5.2 users can register custom conversions to the Qt meta system
using QMetaType::registerConversion(). This commit updated a internal
routine which makes use of it if our own mechanism fails.
Added convenience method MetaObject::of<T> ();

Simple method which uses the Qt meta system to look-up the
Nuria::MetaObject of type T.
Rewrote Nuria::Serializer, now using Nuria::MetaObject

Using the Nuria meta system, serializer is now more powerful in
regards of easily (de-)serializing data structures as custom structures
no longer need to derive from QObject. Further, they now benefit from
features like NURIA_REQUIRE which makes verifying structures really
easy.
Moved stealPointer() utility function from Serializer to Variant

stealPointer() is a hack for QVariant, returning the contained pointer
and making the variant itself invalid. Use with caution.
Mention Variant::stealPointer() in MetaMethod documentation.