Multi section form support

Added support for forms with multiple sections. eg:

var multiSectionForm = forms.create({
    userDetails: {
        username: fields.string({required: true}),
        password: fields.password({required: true})
    },
    address: {
        street: fields.string({required: true})
    }
});

Support for this is already in the forms module. But I wanted to be able to have multi section forms with client side validation.

I added some basic tests and was going to add some more but haven't had the time and I thought I may as well submit this pull request in case others want to use this functionality.