Upload large files (binary)

Allows uploading large files using a configured upload handler.

Progress bars are provided for all themes except jquery-ui, in which case progress is not shown.

Includes examples/upload.html to demonstrate.

Example schema

````javascript
schema: {
  type: "object",
  title: "Image",
  properties: {
    href: {
      type: "string",
      format: "uploadUri"
    },
    name: {
      type: "string"
    }
  }
}
````

Configured upload handler

````javascript
JSONEditor.defaults.options.upload = function(type, file, cbs) {
  ...
}
````

Resolves issue https://github.com/jdorn/json-editor/issues/166