Add `:fieldset` option to `fields_for`.

By default `fields_for` wraps the fields in a <fieldset> tag (which
is styled as a panel).

After this commit, the behavior can be disabled by passing the
`fieldset: false` option to `fields_for`.
Merge pull request #32 from claudiob/optional-fieldset-for-fields-for

Add `:fieldset` option to `fields_for`.