12358 parse metadata

See https://trac.openmicroscopy.org.uk/ome/ticket/12358#comment:1

DV metadata objects were expecting to be processed via the `toString()` method. This no longer holds. Some for of API addition is needed in order to make this more general.

/cc @melissalinkert @mtbc
well, there's the question: in 5.0 we turn all shorts into ints and then in 5.1 add short support?