Adds support for passing JSON parameters to initialize XBlock

This allows this XBlock initialization to pass a JSON argument from Python to JavaScript. This eliminates lots of template hacks getting data into XBlocks. 

There is a matching commit in the XBlock repo. 

@cpennington @rocha 
It did in my editor. ;) 

Fixed. Tabs are now spaces. 

Do keep in mind spacing throughout the file is inconsistent with edX JS coding standards (https://github.com/edx/edx-platform/wiki/Javascript-standards-for-the-edx-platform). Top is two space indent instead of four.