Apply correct access control to GET of non-current rev of a doc

Access should be checked based on the channels that revision is in, not
the doc's current channels. (Fixes #81.)
Speed up unit tests by disabling inclusion of JS 'underscore' library

On my machine, when underscore is enabled, otto.New takes ~80ms instead
of < 1ms. This makes the gateway unit tests take a lot longer (20 sec vs
3 sec.)
Update doc channels/access correctly if update involves a conflict

If there's a conflict, the newly-added revision may not become the
current revision. In that case, we need to get the new current rev's
channels and access to store into the top level of the document object.
Fixes #82