make handles larger for next/prev month

I suggest we do the following, my users were complaining...

  return '<span class="next">'. l('»', $url, $attributes) .'</span>';

with

  return '<span class="next" Title="View the next month">'. l('>>', $url, $attributes) .'</span>';

and

  return '<span class="prev">'. l('«', $url, $attributes) .'</span>';

with

  return '<span class="prev" Title="View the previous month">'. l('<<', $url, $attributes) .'</span>';