Modularize and unit test loading arrows

- Adding arrows.js module file to encapsulate the loading of arrow gifs
- Added url value of arrow logos to all config files
- Importing config.js and arrows.js to widget.html
- Ensuring to load arrows prior to calling `_getData()` in spreadsheet.js module
- Including config/test.js and arrows.js to widget unit test dependencies
- Removed dummy-spec.js and added arrows-spec.js for unit testing the arrows module