Class-ify fields using coffeescript, able to generate AST

@imlucas 
Finally all the field types are abstracted as different classes. Yay!
The program now is able to generate a simple AST from a json object, handle aggregation among fields of the same type, and do some basic counting.
To get an example output, run 'node cli.js' to parse the json objects in test.json.
If the class structure looks good, I can go on adding more functionalities.
Thanks!