title: How add empty sheet to `excel` file that has already created using pandas
body: <p>I have data and I add that to excel </p>
  
  <pre><code>df.groupby('id').apply(lambda g: g.to_excel(g.name + '.xlsx', u'Смартфоны кратко'))
  </code></pre>
  
  <p>Next I add <code>sheet</code> with data to those files</p>
  
  <pre><code>df_upd.groupby('id').apply(lambda x: add_xlsx_sheet(x, u'Смартфоны полно', path='{}.xlsx'.format(x.name)))
  </code></pre>
  
  <p>But how I can add empty <code>sheet3</code> to those files? 
  df_upd looks like </p>
  
  <pre><code>                 date                mail         browser
  3641   2016-06-14 15:13:36  yans.bouts@yandex.ru  unknown   
  3660   2016-06-14 15:13:38  yans.bouts@yandex.ru  unknown   
  3666   2016-06-14 15:13:39  yans.bouts@yandex.ru  unknown    
  104298 2016-06-15 20:16:54   mgt198911@gmail.com  unknown   
  104308 2016-06-15 20:16:54   mgt198911@gmail.com  unknown   
  104367 2016-06-15 20:18:14   mgt198911@gmail.com  unknown   
  104410 2016-06-15 20:18:17   mgt198911@gmail.com  unknown   
  104464 2016-06-15 20:18:19   mgt198911@gmail.com  unknown
  </code></pre>
  
  <p>And <code>add_xlsx_sheet</code> is</p>
  
  <pre><code>def add_xlsx_sheet(df, sheet_name=u'Смартфоны кратко', index=True, digits=2, path=None):
  book = load_workbook(path)
  writer = pd.ExcelWriter(path, engine='openpyxl')
  writer.book = book
  writer.sheets = dict((ws.title, ws) for ws in book.worksheets)
  if sheet_name in list(writer.sheets.keys()):
      sh = book.get_sheet_by_name(sheet_name)
      book.remove_sheet(sh)
  df.to_excel(excel_writer=writer, sheet_name=sheet_name, startrow=0, startcol=0,
              float_format='%.{}f'.format(digits), index=index)
  writer.save()
  </code></pre>