Use @id instead of custom ID properties

[trieloff]

As we rely on JSON-LD semantics more heavily (see also #43), I thought it made sense to follow JSON-LD conventions when it comes to IDs and:

• use the @id property to identify the current object, instead of in addition to *_id properties that differ from schema to schema
• use URIs (specifically IRIs) to identify objects instead of having an off-band resolution mechanism

This pull request applies this convention to the asset schema category.