[#70] lazy-load metadata instances
[#70] lazy-load metadata instances

- Instead of pre-loading `Metadata` instances, create them only when
  they are retrieved. Change is 100% BC with previous version.

Cube-Solutions/zf-hal,Wilt/zf-hal,Cube-Solutions/zf-hal,vixriihi/zf-jsonld,Cube-Solutions/zf-hal,nobesnickr/zf-hal