This is a working version that allows users to get the content of BitFiles.
It's a bit bad because it draws the whole stream into memory and then
serves it up to the client.
The tempfile alternative didn't work so well - the way rails renders makes
it hard to ensure that the tempfile is deleted when we're done with it
but not before.  (Specifically it seems to delegate serving the file to
rack, but the controller action, where we'd delete the Tempfile, seems
to finish _before_ rack serves up the file. And there doesn't seem to
be a clean way around that.)
I'm checkpointing here to see if I can find some sort of buffered IO
to use in place of the StringIO. I don't know if that will interoperate
correctly with Mechanize's retrieval of the data, but it's worth a shot.
Got serving bit files to work using temp file.