Add form validation example

[sriharish]

This pull request removes the existing credit-card-form example and adds a general form validation example for registering a new user as per #1148.

What was Removed

• Removed examples/credit-card-form/* as suggested by the comment here

What was Added

• Added example program for validating "new users" in form-validation/main.go.
• It features error validation messages that appear below the form (and only once the form submission has been attempted at least once with a keypress enter after signature).
• The errors are removed as the user corrects the text inputs.
• Submission (on keypress enter when the cursor is on the last text input) is prevented if errors persist.
• A border style was added around signature for aesthetic purposes

With Errors

Successful Submission