AdamW: epsilon and lambda swapped?

[murrellb]

Weird, given that the docs imply epsilon is non-zero:

But looking at the code, the field order in the struct vs the constructors seems to swap them?

[ToucheSir]

I think you may be right, but cc @CarloLucibello for another pair of eyes.

[CarloLucibello]

omg, that's a huge oversight. fix in #198