A write stream that calculates SHA from the output and updates the manifest accordingly.  manifest - Manifest object filename - Filename (manifest property to set) output   - Pipe to this output stream
function SHAWriteStream(manifest, filename, output) {
  this.output = output;
  this.manifest = manifest;
  this.filename = filename;
  this.sha = Crypto.createHash("sha1");
  output.on("close", this.emit.bind(this, "close"));
  output.on("error", this.emit.bind(this, "error"));
}