Pandas Series should provide to_excel method
Hello,

Pandas Series provide `to_csv`method but they doesn't provide `to_excel` method.
Adding `to_excel` will avoid user to convert a `pandas.core.series.Series` to a `pandas.core.frame.DataFrame` in order to output Excel file

DataFrames provide 

```
df.to_clipboard  df.to_hdf        df.to_period     df.to_string
df.to_csv        df.to_html       df.to_pickle     df.to_timestamp
df.to_dense      df.to_json       df.to_records    df.to_wide
df.to_dict       df.to_latex      df.to_sparse
df.to_excel      df.to_msgpack    df.to_sql
df.to_gbq        df.to_panel      df.to_stata
```

Series provide

```
ts.to_clipboard  ts.to_frame      ts.to_period     ts.to_string
ts.to_csv        ts.to_hdf        ts.to_pickle     ts.to_timestamp
ts.to_dense      ts.to_json       ts.to_sparse     ts.tolist
ts.to_dict       ts.to_msgpack    ts.to_sql
```

Kind regards
