Add hidden input and custom view layout

This PR adds jQuery in order to allow DOM inserts to evaluate script tags.

Also, it adds a new `view` class method in the Form, that allows to specify a custom view (actually a layout) that lets you define custom content in the form modal, and with `yield` you then include the actual inputs.

This also adds support for `hidden: true` option in the input, which allows to render hidden fields in the form.

Lastly, I restructured a little bit the spec_helper and stuff and added a dummy Rails app in order to test the engine with all its controllers, helpers and stuff.

Let me know what you think of this!