Include all modtimes in the manifest, only include files if newer

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 (closed).