[WIP] Form builder helpers

- [x] Move mixins to `lib/angular_sprinkles/mixins`
- [x] Create `f.bind_*` helpers to auto bind properties with `ng-model` in forms
- [ ] Feature spec for all helper methods.
- [ ] Create `ng_form_for` for auto-binding a service function to a form submit. e.g.
```erb
<%= form_for @model do |f| %>
  <%= f.label :name %>
  <%= f.bind_text_field, :name %> <%=# changes to this field are bound to @model %>

  <%= f.text_field :note %> <%=# this is not bound, but still part of the form %>
<% end %>

<%= ng_form_for @model, :handle_submit_service do |f| %>
  <%=# when the form is submitted, the handleSubmitService will execute %>
  ...
```