Creates a new Zip file.  zip      - Zip object filename - Filename modified - Modified date (optional)
function File(zip, filename, modified) {
  this.zip = zip;
  this.filename = filename;
  this.modified = modified || new Date();
  // True while file is writeable (end/destroy change this)
  this.writable = true;
  // We use this to hold any buffer if this is not the active file
  this._buffers = [];
  // Make sure we only write header once.
  this._wroteHeader = false;
  // True if file fully written out; distinct from fully read (writable = false)
  this._done = false;
  // Offset of file within the stream
  this._offset = 0;
  this._crc = 0 ^ -1;
  this._compressedLength = this._uncompressedLength = 0;

  // Write to _deflate, count data size before passing to output.
  var file = this;
  this._deflate = Zlib.createDeflateRaw();
  this._deflate.on("data", function(buffer) {
    file._compressedLength += buffer.length;
    zip._writeBuffer(buffer);
  });
  this._deflate.on("end", function(buffer) {
    file._doneWritingFile();
  });
}