spec: clarify ACI format and signing/encryption
spec: clarify ACI format and signing/encryption

This patch attempts to be much more explicit and strict about what an
ACI file actually is and tighten up a lot of the language.
- Be clearer about ordering of encryption/signing
- Move towards RFC2119 wording ("MUST", "MAY", "SHOULD").
- Breaks out Image ID into its own section to reflect that it is a first
  class concept.
- Remove second person references

ChengTiesheng/spec,3ofcoins/appc-spec,iaguis/appc-spec,marineam/spec,jonboulle/spec,appc/spec,yifan-gu/spec,appc/spec,stufflebear/spec,krnowak/appc-spec,ChengTiesheng/spec,dustymabe/spec,zenlinTechnofreak/spec,appc/spec,tklauser/spec,apcera/appc-spec,endocode/appc-spec,alban/appc-spec,marineam/spec,zenlinTechnofreak/spec,stufflebear/spec,dgonyeo/spec,apcera/appc-spec,krnowak/appc-spec,craimbert/spec,endocode/appc-spec,jonboulle/spec,endocode/appc-spec,marineam/spec,3ofcoins/appc-spec,craimbert/spec,iaguis/appc-spec,apcera/appc-spec,jonboulle/spec,tklauser/spec,krnowak/appc-spec,dgonyeo/spec,stufflebear/spec,yifan-gu/spec,ChengTiesheng/spec,dustymabe/spec,squeed/spec,dustymabe/spec,alban/appc-spec,squeed/spec,craimbert/spec,zenlinTechnofreak/spec,yifan-gu/spec,alban/appc-spec,3ofcoins/appc-spec,iaguis/appc-spec,squeed/spec,dgonyeo/spec,tklauser/spec