Merge pull request #6 from crowdint/popup_adjustment

* Fix the problem displaying the popup after it was displayed once.
Bugfix subscription price
* Add spanish translation for interval options.

- Add i18n translations
- Display translated interval options
- Prevent add duplicated interval options
Merge pull request #7 from crowdint/translate_options_interval

* Add spanish translation for interval options.
Bugfixing

- Fix bug deleting Spree::SubscriptionInterval
- Prevent creating negative subscription interval
Merge branch 'master' into bugfixing
Merge pull request #8 from crowdint/bugfixing

Bugfixing
Merge branch 'master' into popup_adjustment