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()