add course tree example

I only changed the json from the example at d3 website. Many changes need to be made to this example:
* The nodes are sorted alphebatically right now, but should be changed to no sorting, so that "2009 Spring" should be listed before "2009 Fall"
* I don't know how "size" value is used (need to go over the source code)
* we need to create a more complete structure for course json file to include other information such as description, instruction, credits, etc.