Validation for S3 UI fields

Closes #30 

Uses the new i18n system for error messages, and HTML5 form validation attributes.
What do you think about using this: http://rickharrison.github.io/validate.js/

It still gives us inline validation like the native HTML5 forms, but allows arbitrary validation functions rather than single regexes (good for bucket and region), and has no dependencies (unlike Parsley which needs jQuery).