Initial implementation of the json parser

This patch adds initial support for the json response type from intuit. It adds a objectifier class which lazily converts the json response into objects.

One other change is the tag pattern regex for converting camelCase to underscore_case, I changed it to take into account "401K" which originally was being converted to "401_k", and now is converted to "401k".

I've only written basic tests to make sure the objectifier adheres to the same interface as the original xml implementation

Partial fix for #6
Closes #7