Firming up a bunch of details

Fixed an Actor-communication bug between PhotoController and
PhotoUploadActor.  (Lesson to me, that I need to be careful about
composing Futures.)

Pass the propId, and use it for constructing values properly.

Introduced the formal concept of Collection.append(), which represents
the notion of "adding" a new value to a Collection.  Roughly corresponds
to Seg.:+() in Scala.

Building the new QValue, to store into the Space.  (Although it isn't
stored yet.)

Removed the clever overload of ByteArrayOutputStream's toByteArray() --
it was resulting in unnecessarily long files.