separate external and internal verification APIs

[monkeypants]

in #9 @onthebreeze suggested separating the notary action from the verification API. I'd like to make a similar suggestion; separate the external verification API (this is how you verify notarised documents/deeds) from the internal verification API (this is how you implement a different back-end in our library).

It's a separate question if the internal API actually belongs as part of the spec, or if it belongs in the TradeTrust reference implementation spec.