Add basic form validation styling

[jlandfried]

Right now out of the box there's no styling for basic form validation elements, it'd be nice to not have to start from scratch with this on each project.

At the very least we could handle:

• Labels for required fields
  • Just an asterisk to the right of the label (span.form-required)?
• Indication for inputs that don't pass form validation
  • Just a red outline around the input with a .error class?

Feel free to add any other things that might be useful here, but these are the ones that jumped out at me.