Add support for hidden fields

This PR adds support for hidden fields. This should fix #65.

Validation errors are not added to unbound hidden fields in the get_field_errors method. One known problem with this implementation is that if there actually are errors (added server-side by django core), they will not be rendered properly (see discussion on #65)

On the angular side, this sets the initial value for hidden fields as in #60, which was reverted in a9e240b8a6c0f16bc327fb76eee444dc7ffb58eb. I am certainly open to discussion on other approaches to this, but this seems like a sensible way to make the value available on the scope.