Preprocessing of sent140

When preprocessing sent140, the intermediate `.csv` file saved by `combine_data.py` will have blank lines, causing `data_to_json.py` to fail to run. 

In addition, an error in the encoding format will also be reported. 

It is suggested to change line `27` of `combine_data.py` into the following form:

``` python
with open(out_file_name, 'w', encoding='ISO-8859-1', newline='') as f:
```