Add download data file script

I add `download_data.py` script here. And I move all scripts from /scripts folder to /zipgun/scripts, and make it a python package. In this way, you can call the script like this

```shell
python -m zipgun.scripts.download_data
```

For generating db, you can also do

```shell
python -m zipgun.scripts.generate_db
```

I prefer this way to call scripts of a package, see if you like it. Feel free to change back if you don't like this approach.

Ref: #1