Merge branch 'release/2.5.3' into develop
2014 pricing, international rates updated too
Added Helper test
Moved world zone codes into protected class vars, and replaced switch with if/else
Helper improvements, comments and more descriptive method names
Bug fix: additional weight charges helper didn't calculate correctly
Amended comment
Merge pull request #9 from ashsmith/feature/march2014pricing

2014 pricing, international rates updated too
Bump the version number
Merge branch 'release/2.6.0'