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
Changed Heroku Button's template
Oops!
Fix crawler to skip duplicated items correctly
Merge pull request #201 from iwamot/fix_dup_items

Fix crawler to skip duplicated items correctly
Fix missing api key
Add .DS_Store to .gitignore
Merge pull request #202 from iwamot/fix_js_error

Fix missing api key
The scheme of DATABASE_URL for postgresql is "postgres"
change Gemfile to sorted style

* sort gems by ascii
* prefer double quotation instead ob single quotaton
* use sub-group style for development/test group gems

cf. https://gist.github.com/kakutani/43b9f42197ab002fcdf8
remove unused sub-group