By generating the manifest entry before we determine whether to include
the file in the archive, we ensure that every file is represented in the
payload to the server.

This allows the server to make a decision on whether to supersede a tile
in its output archive.

In addition, this commit introduces a small optimization in the "new
tile" path: we no longer force PIL to parse the entire image just to
re-encode it and write it back out to disk. Data is copied directly from
the incoming archive. This places some amount of trust in the client.

Fixes #4.