Add AWS validator class

This closes #30.

This class contains four methods for validating AWS credentials, which can be used to do an initial syntax check on the UI script before they are sent to the background script.
Okay, I like the idea of setting the pattern attribute programmatically from the validator.
In terms of feedback for invalid parameters, do you know how to get that to display? It would be nice if it rejected the button click automatically (for bucket name and region at least) if the contents are invalid.

I'm sure HTML5 has this but I can't get it to work.