refactoring database.yml, mail.yml, and *.env files

* add `database.yml`, `mail.yml` to git repository
* the environment variables in each config files are defined in `config/environments/development.env`
* revise README.md

You can set the default environment variables only by copying two files.
```
cp config/environments/development.env.example config/environments/development.env
cp config/environments/test.env.example config/environments/test.env
```