0.2.0 Threaded Workers

This update introduces an official way to handle threaded jobs to the library with Workers and LiveWorkers! For implementation details see https://puzzlepiece.readthedocs.io/en/latest/puzzlepiece.threads.html and for an example of how to use the new PuzzleTimer and the returned Signal, see https://github.com/jdranczewski/puzzlepiece/blob/v0.2.0/puzzlepiece/pieces/plotter.py

Some convenience functions were also added to the Puzzle object, see 7319dbd