Implement Save/Load using pickle #120
Description
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.