Refactor to use I18n#getOrKey
version 1.2.8

- [blank-app] Bump sbt-launch.jar to 0.13.5
- [blank-app] Remove baseSettings from scalatePrecompileSettings
- [blank-app] Bump sbt-scoverage to 0.99.7.1
- [common] Add I18n#getOrKey methods
- [framework] SkinnyApiResouce's #createApiAction should be protected
- [framework] Add nested i18n message support
- [task] Update scaffold generator to use I18n.getOrKey instead of I18n.get
- [validator] Enable some built-in validators dealing with empty values
- [validator] Improve regexp for validation