New example for generating JSON (as well as source JSON file)

Added a new example to demonstrate how to actually generate JSON, how to use yajl_gen_status codes, and how to get access to the buffer after you are done generating. I also included public_twitter_timeline.json, since we are creating the first object in that JSON file, in case anyone wants to see the original to visually see what is going on in gen_example.c

I have not done any work for this example to be processed or whatever by doxygen , or to add it as a build target or any of that, as I am not familiar with cmake / doxygen.