collections spec

[will-moore]

This follows discussion of a Collections spec at #31

This contains the core of what was agreed in the issue above and omits groups, layouts, transforms etc.

This PR also includes the validation logic for json_schema within a new 0.4 directory (although the version for collections may be later). The test allows for different schemas to be used to validating samples under 0.4/examples/image/ and 0.4/examples/collection/. The samples have been copied over from 0.3 with the expectation that these can be updated as the 0.4 spec PRs are merged.

[joshmoore]

#69 is now merged with various investigations into validation & subclassing options for collections.