Make database adapter selectable for production

- Install both `mysql2` and `pg` gems by default
- Add `BUNDLE_WITHOUT` option in app.json
- Update setup instructions in README.md
- Fix typo: database.yml.potgresql -> database.yml.postgresql
Select db adapter based on configuration or DATABASE_URL
Copy database.yml before build
Copy database.yml on build
Copy database.yml before/in build
Include sqlite3 on development/test
updated Gemfile.lock
remove traial code
Merge pull request #199 from ayumin/db-adapter

Select database adapter based on configuration