Set PostModel defaults, and set locale when creating post

Set default values for post locale and status.
Hard code post locale value.
This should ensure posts now get saved!
Add validation using backbone-validation

Add post model validation using backbone-validation. This seems to work
with backbone-forms perfectly. We now check errors and only save the
model if
validations passes. We're not handling server validation errors yet,
but we are
only closing the modal if the post is saved.
Fix image paths in test css file
Wiring up custom form fields

- Add backbone deep-model for easy access to nested post attributes.
- Generate HTML for custom form fields.
- Update modal controller to loads post relations (ie. form) before
rendering.
- Add getPostSchema() and getPostFieldsets() to FormModel - these turn
attributes and groups into schema and fieldset objects for
Backbone.Forms
- Turn PostModel.schema into a function, and merge custom form fields
into post schema(), add post fieldsets() to return full fieldset list.
- Hacked backbone-forms to replace template for nested forms and avoid
nested <form> elements.