Issue #2327283 Create translations for a currency

- Refactor commerce_price.install to use the currency importer service.
- Rename the importer service.
- The Symbol field is translatable as well.
- Create translations for currency entities when a new language is added or when the user adds
  a new currency.
Merge pull request #67 from Jbekker/currency-translations

Issue #2327283 Create translations for a currency