SPEC : cleanup
Merge pull request #39 from rahulgopan/master

spec: typo fix (images -> image)
README: update to reflect actool is now quiet by default
Merge pull request #41 from jonboulle/quiet

README: update to reflect actool is now quiet by default
spec: default digest format is sha512

Currently the ACI spec leverages sha256 as the default digest format, which
is not as performant on 64-bit systems. A modern 64-bit processor can compute
a SHA-512 hash in about 50-75% of the time it takes to create a SHA-256 hash.
(This is because SHA-512 primarily uses 64-bit data types.) Considering that
container images can become rather large, this is substantial.

Fixes #8
Merge pull request #24 from rckclmbr/sha512

Use sha512 instead of sha256
schema: expose functions to retrieve label values
schema: expand OS/arch whitelist to allow other platforms

Introduced a map of valid OS/architecture combinations to allow
implementations supporting platforms other than linux/amd64. I can't
figure out at the moment how to warn rather than fail during JSON
validation without resorting to "just print to stderr" approach. A
workaround for an implementation would be to add own values to the
`ValidOSArch` map, but this would still not allow full flexibility.

Additionally fixed a typo in Labels.UnmarshalJSON that prevented the
validation from actually being executed (an empty `l` Labels were
validated, rather than deserialized `nl`).

Improves #6 (not a full fix, as unlisted combinations still fail)
Merge branch '3ofcoins/expose-labels' into for/jetpack

Conflicts:
	schema/types/labels.go