Add metadata schema

[chuwy]

We currently return all schemas with metadata key (if requested with ?metadata=true) which contains creation and modification dates. @mikejongbloet would like to add more properties such as owner and type (event/context) and in order to make this info generic and snowplow-independent we can come up with metaschema that validates these kind of properties. These schemas could reside independently on Iglu Server themselves.