Implement Save/Load using pickle

[csala]

Current save/load methods focus on dumping and loading the pipeline definition in its JSON form, but provide no means to save a fitted pipeline and load it later to make predictions, being the usage of pickle outside of the pipeline the only way to go.

Let's re-implement the save/load methods to save the whole pipeline instance, and move the current save functionality to a to_json method.

Note that from_json is not needed anymore because the pipeline annotation can already be passed as a JSON path in the pipeline __init__ method.

[WALEX2000]

Hey, it's been quite some time since I've seen any updates, and I was currently looking into this issue precisely. Is this project dead in the water?