Additions:

* Go package added for types
https://godoc.org/github.com/opencontainers/image-spec/specs-go/v1 (#172)
* oci-image-tool: man page docs added
https://github.com/opencontainers/image-spec/tree/master/cmd/oci-image-tool/man
(#180)

Minor fixes and documentation:

* oci-image-tool: replace colon with hyphen to be windows friendly (#177)
* oci-image-tool: fix various bugs in extraction of images (#177)
* manifest: size as int64 allowing for huge containers (#153)
* manifest: move duplicate descriptions to reference descriptor doc (#167)
* serialization: explain opaques and their role (#171)