Parse attachment ids out of the attachment pointer url and return them
as strings because the copy parsed by JSON suffers a loss of precision.
Convert them to and from the format expected by the protobuf using
facilities from decodeIO.Long.
DRY up protobuf declarations and move to a slightly briefer naming
convention.
Also dropped some ArrayBuffer -> string conversions as
ProtoBuf.js handles ArrayBuffers just fine, and in fact, more
efficiently than strings.
Finally, dropped the btoa() wrappers, because that incurs an extra
string -> string conversion before the protobuf's internal string ->
array buffer conversion. In lieu of btoa, we can simply pass in the
optional string encoding argument to the protobuf's decode method,
which in these cases should be 'binary'.
Related: #17
All the group messages were being sent to the last recipient in the
list, due to the persistence of `var number` in later loops and async
calls. An easy mistake to make, when you use for instead of each.
