IFRAME-based file uploads in Model.sync

Add an implementation of Model.sync that can upload files using
a hidden IFRAME. This provides support for syncing models with file
uploads in browsers like Internet Explorer <10 that don't support
``FileReader`` or ``FormData``.

This relies on the jQuery Iframe Transport plugin.  I like this
method because we can conditionally load the plugin only when
file API support isn't present. Similarly, we just have to update
a few options before delegating to ``Backbone.sync``.

Addresses #595