0.9.0: MinMaxNormalizer changes.

[MathisWellmann]

• Renamed HlNormalizer to MinMaxNormalizer
• Fixed future data leakage concern in implementation which included the last values in the min-max values before returning the final value. Now the newest value is normalized against the current high-low range, then the range is updated
• Don't return values when less than window_len data samples have been seen, eliminating the warmup transient response.