schema/types: add Hash.Set() UUID.Set(), and Hash.Empty()

Set() in addition to String() fulfills the flag.Value interface enabling
convenient commandline parsing for these types as Var flags.

Hash.Empty() is just useful like UUID.Empty(), I'm presently using them
check if variables of these types have been set to anything as flags.
Merge pull request #38 from vcaputo/uuid-hash-setters

schema/types: add Hash.Set() UUID.Set(), and Hash.Empty()
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
Use sha512 instead of sha256

Fixes #8
Clarify what maxHashSize is