Validation for S3 UI fields

Closes #30 

Uses the new i18n system for error messages, and HTML5 form validation attributes.
You don't have to escape hyphen in this context.
Also this validation doesn't check the "Bucket names must not be formatted as an IP address (e.g., 192.168.5.4)." and "There can be only one period between labels."
http://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html
Is it intentional?

Because the official document determines the strict rules about the bucket name, it might be okay to prevent proceeding if bucket name is invalid.