code: @Override
      public Promise<Void> chmod(String path, String perms) {
          return adapter.toPromise(handler -> vertx.fileSystem().chmod(path, perms, handler));
      }
docs: Change the permissions on the file represented by {@code path} to {@code perms}, asynchronously.
  <p>
  The permission String takes the form rwxr-x--- as
  specified <a href="http://download.oracle.com/javase/7/docs/api/java/nio/file/attribute/PosixFilePermissions.html">here</a>.
  
  @param path  the path to the file
  @param perms the permissions string
  @return a promise for completion