Fix a scoping issue and add some minor features. Fixes #1.

- A scoping issue was preventing embedded `ngForm`s from working. This is fixed now.
- The parent form will tell you which embedded `ngForm`s have errors. You can specify a nice name with a `form-nice-name` attribute on the `ngForm`.
- You can override the form it uses to display errors by passing a form in via the `form` attribute on the `formErrors` directive.