v1.21 Improve optimizer selection, input signature

FEATURES:

• from keras_adamw import now accounts for TF 1.x + Keras 2.3.x case
• model and zero_penalties now show up in optimizer constructor input signatures, making them clearer and more Pythonic
• Each optimizer now has its own full docstring, instead of deferring to help(AdamW)

BREAKING:

• model is no longer to be passed as first positional argument, but as a later one, or a keyword argument (model=model)

BUGFIXES:

• name defaults corrected, many were "AdamW" even if not AdamW - though no bugs were encountered as a result

MISC:

• __init__ wrapper moved inside of __init__ to avoid overriding input signature